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Nickel-doped (Zr0.8, Sn0.2)TiO4 for microwave and millimeter-wave applications
Materials Science and Engineering B 118 (2005) 205–209
Nickel-doped (Zr0.8, Sn0.2)TiO4 for microwave and millimeter-wave applications A. Ioachima , M.G. Banciua , M.I. Toacsana , L. Nedelcua , D. Ghetua , H.V. Alexandrub,∗ , G. Stoicac , G. Anninod , M. Cassettarid , M. Martinellid a
National Institute of Materials Physics, Bucharest-Magurele, Romania b Faculty of Physics, University of Bucharest, Romania c S.C. IPEE S.A., Curtea de Arges, Romania d I.P.C.F.-C.N.R., Pisa, Italy
Abstract (Zr0.8 , Sn0.2 )TiO4 ternary compounds (ZST) have been prepared by conventional solid-state reaction from raw materials. The effects of such sintering parameters as sintering temperature, sintering time, and NiO addition on structural and dielectric properties were investigated. The material exhibits a dielectric constant εr ∼36.0 and high values of the product Qf of the intrinsic quality factor Q and the frequency f from 32,170 to 50,000 at microwave frequencies. The dielectric loss tan δ values of ZST ceramics are decreased by low-level doping of NiO, while the temperature coefficient of the resonance frequency τ f takes values in the range −2 to +4 ppm/◦ C. Investigations on whispering gallery modes revealed low dielectric loss in millimetre-wave domain. An intrinsic quality factor of 480 was measured at 115.6 GHz. Dielectric resonators and substrates of ZST material were manufactured. The dielectric properties make the ZST material very attractive to microwave and millimeter-wave applications, such as dielectric resonators, filters, planar antennas, hybrid microwave integrated circuits, etc. © 2005 Elsevier B.V. All rights reserved. Keywords: High dielectric constant; Dielectric resonators; Microwaves; Millimeter-waves
1. Introduction The development of modern microwave and millimeterwave communication systems requires new materials with appropriate properties [1] in specific frequency ranges. For many applications, dielectric materials with low loss and high dielectric constant [2,3] offer a high degree of miniaturization and improved performances [4]. Dielectric materials based on the ZrO2 –SnO2 –TiO2 ternary system [5] are very attractive for applications in the microwave domain due to their high dielectric constant, low loss, and controlled temperature coefficient of the permittivity [1,3]. Such compounds as (Zr0.8 , Sn0.2 )TiO4 exhibit an almost zero temperature coefficient and offer a great temperature stability to the specific applications such as frequency ∗ Corresponding author. Present address: University of Bucharest, Faculty of Physics, P.O. Box 74-165, Bucharest, Romania. Tel.: +40 21 413 3367; fax: +40 21 413 3367. E-mail address: [email protected] (H.V. Alexandru).
0921-5107/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2004.12.071
discriminators [4], low phase-noise dielectric resonator oscillators (DRO) [6], duplexers, and filters [1]. These materials are generally difficult to sinter without additives, especially at lower temperatures. The sinterability increases with additions, such as La2 O3 , ZnO, NiO, but no mechanisms have been proposed for the improved densification kinetics or for the addition effect on the dielectric loss. The La2 O3 addition has been reported as an improving sintering factor, which promotes the grain growth [7].
2. Experimental The (Zr0.8 , Sn0.2 )TiO4 ceramic materials were prepared by standard solid-state reaction technique. Powder oxides ZrO2 , SnO2 , and TiO2 (equivalent to a weight ratio of 47:15:38) with purity higher than 99.9% were mixed according to the (Zr0.8 , Sn0.2 )TiO4 stoichiometry. In order to reduce the sintering temperature, 2 wt.% La2 O3 and 1 wt.% ZnO were added.
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The powders were milled in the distilled water for 24 h in a mill with agate balls. All mixtures were dried and treated at 1200 ◦ C for 2 h. The calcined powders were then milled again for 2 h. For some samples, 0.2 wt.% NiO was added to the calcined powders, in order to investigate the effect on sintering properties of ZST. Pellets of 10 and 5 mm diameter were formed by uniaxial pressing and then sintered at 1280–1400 ◦ C for 2–4 h. The morphology, phase-composition, and microstructure of the sintered ceramics were analyzed by using scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) microanalysis. The crystalline phases were identified by X-ray diffraction (XRD) patterns. The bulk densities (ρr ) of the sintered pellets were measured by the Archimedes’ method. The structure of ZST was investigated by X-ray diffraction, Seifert Debye Flex 2002 diffractometer, provided with copper target X-ray tube (λ (Cu K␣ ) = 0.1541 nm). At microwaves, the ZST cylindrical samples exhibit very low dielectric loss, high dielectric constant, and a very good stability with temperature. Therefore, the dielectric parameters of ZST samples were investigated by using the Hakki–Coleman method [8]. A screen plot of the signal attenuation versus frequency is shown in Fig. 1 for a ZST sample with 5.94 mm height, 9.05 mm diameter, and εr = 36.6 dielectric constant. Each peak in Fig. 1 corresponds to a transversal electric, TE0np transversal magnetic TM0np or a hybrid electromagnetic HEMmnp resonance mode [1] where m, n, and p are the azimuthal, radial, and axial indices, respectively. The cursor C indicates the TE011 resonance mode, which is used in the Hakki–Coleman method [8]. A computer-aided measurement system combining a HP 8757C network analyzer and a HP 8350B sweep oscillator was employed in the microwave measurements. The temperature coefficient τ f of the resonant frequency in microwave range was measured by heating the samples from +18 to +90 ◦ C. The dielectric loss in millimetre-wave range was examined by investigating the whispering gallery modes (WGM) [9,10] of the samples with diameters between 5 and 10 mm and height between 2 and 4 mm. These modes were excited by
Fig. 1. The plot of the HP 8757C network analyzer screen when measuring a ZST cylindrical sample with 5.94 mm height, 9.05 mm diameter, and 36.6 dielectric constant. The attenuation vs. frequency curve exhibits resonance peaks corresponding to the resonant modes. The cursor C indicates the TE001 mode.
using a dielectric waveguide as shown in Fig. 2. The whispering gallery modes (WGMs) exhibit a strong confining of the electromagnetic field into the dielectric disk. Therefore, the WGMs allowed dielectric measurements in the millimeterwave range due to their very low radiation loss. An example of the WGMs resonating in a disk ZST is illustrated in Fig. 3. Each resonance dip shown in Fig. 3 corresponds to a WGM.
3. Results and discussions The effect of the sintering temperature Ts on ZST samples was analyzed. The investigated samples were generally multiphase. The X-ray diffraction patterns, which are presented in Figs. 4 and 5, showed that the (Zr0.8 , Sn0.2 )TiO4 compound was the majority phase, corresponding to the standard crystalline data [14]. At lower sintering temperatures, e.g. Ts = 1330 ◦ C, the minority phases, such as ZrTiO4 , and a
Fig. 2. Experimental setup for ZST disk characterization in the millimetre-wave range.
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Fig. 5. XRD pattern of ZST samples without NiO sintered at 1360 ◦ C/4 h.
Fig. 3. Amplitude of the reflected signal in the millimeter-wave range vs. frequency, showing whispering gallery modes (WGM) for dielectric loss measurements.
very small amount of unreacted TiO2 , were also present. For sintering temperatures Ts equal to or higher than 1360 ◦ C, the ZrTiO4 phase and TiO2 tend to disappear, and the crystalline lattice become more ordered. The X-ray diffraction peaks of (Zr0.8 , Sn0.2 )TiO4 are presented in Figs. 4 and 5 for samples sintered for 4 h at 1330 and 1360 ◦ C, respectively. It can be noticed that the increase of the sintering temperature Ts leads to higher and narrower XRD peaks, thus, to larger crystallites and a better-formed crystalline lattice. The XRD peaks do not change their positions when Ts is changed. Furthermore, the XRD peaks, which are not indexed in Figs. 4 and 5 and which correspond to the minority phases, decrease in intensity with the increase of the sintering temperature. This
effect shows an improved reactivity of the oxides at higher sintering temperature (Fig. 5). The increase of the grain sizes with the sintering temperature increase was confirmed by the SEM observations as shown in Fig. 6. When Ts increases, the grains become more faceted. For sintering temperatures Ts higher than 1400 ◦ C, some intergranular pores with polyhedral facets appear. They can be attributed to the SnO2 segregation at grain boundaries, followed by vaporization at higher temperatures. At temperatures greater than 980 ◦ C, SnO2 decomposes in metallic Sn and O2 . The metallic Sn has the tendency to diffuse out of the grains and to form white-colour spherules during the furnace cooling-down to room temperature [15]. The dependence of the unit-cell volume V0 and the EDX compositional data of the ZST ceramics on sintering temperature Ts is presented in Table 1. Samples 1 and 2 in Table 1
Fig. 6. SEM image of ZST ceramics. (a) Ts = 1330 ◦ C/4 h; (b) Ts = 1360 ◦ C/4 h.
Fig. 4. XRD pattern of ZST samples without NiO sintered at 1330 ◦ C/4 h.
Table 1 The unit-cell volume V0 and EDX compositional data of the (Zr0.8 , Sn0.2 )TiO4 ceramics in relation with the sintering temperature Ts Sample
Sintering temperature Ts (◦ C)
Sintering time tp (h)
Zr (wt.%)
Sn (wt.%)
Ti (wt.%)
a0 (nm)
b0 (nm)
c0 (nm)
V0 (nm3 )
1 2 3
1330 1360 1400
4 4 4
54.62 54.22 45.50
15.50 14.83 16.50
29.89 30.90 36.40
0.476 0.470 0.457
0.550 0.552 0.554
0.502 0.501 0.505
0.1315 0.1301 0.1279
a0 , b0 , and c0 are the unit-cell parameters and V0 is the unit-cell volume; wt.%: percent weight concentration of the element.
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Table 2 Dependence of microwave dielectric parameters on the sintering temperature Ts Sample
Sintering temperature Ts (◦ C)
Sintering time tp (h)
Resonance frequency f (GHz)
Dielectric constant εr
Dielectric loss tan δ (×104 )
Quality factor Q
Product Qf (GHz)
1 2 3
1330 1360 1400
4 4 4
6.09 6.03 6.78
35.30 33.90 35.34
5.5 3.7 1.9
2000 3000 5200
12,180 18,100 35,260
Table 3 Dependence microwave dielectric parameters on the NiO content and sintering time for the samples sintered at 1330 ◦ C Sample
NiO (wt.%)
Sintering time tp (h)
Bulk density ρ (g/cm3 )
Resonance frequency fres (GHz)
Dielectric constant εr
Dielectric loss tan δ (×104 )
Quality factor Q
Product Qf (GHz)
4 5 6 7 8 9
0 0 0 0.2 0.2 0.2
2.0 2.25 2.50 2.0 2.25 2.50
5.02 5.05 5.00 5.03 5.01 4.995
6.653 6.624 6.690 6.740 6.710 6.820
36.8 36.6 36.17 36.33 36.8 36.07
2.14 2.91 2.62 1.45 1.32 2.12
4672 3436 3816 6896 7575 4716
31,080 22,760 25,530 46,480 50,830 32,160
are rich in Zr, compared to the initial weight composition ratio. For sintering temperatures Ts ≥ 1400 ◦ C, the XRD patterns indicate the disappearance of the unreacted TiO2 [11]. The EDX data presented in Table 1 show the decrease of the volume V0 of the unit cell with the increase of the sintering temperature Ts . The Sn ions substitute the Zr ions or Ti ions to a certain degree. The decrease in the unit-cell volume V0 , while the sintering temperature Ts increases, suggests the preferential replacement of the Zr ions by the smaller Sn ions. This fact corresponds to a better short-range ordered structure [12]. The product Qf of the intrinsic quality factor Q and the frequency f of the samples with additions of La and Zn oxides, but without NiO, increases considerably with the sintering temperature Ts as shown in Table 2. The effect of sintering NiO addition on ZST characteristics was also investigated. The Ni ions, like Zn ions, do not diffuse into the grains, but remain at the boundary phase and form a spinel structure (Zn, Ni)2 TiO4 [13]. The SEM images in Fig. 7 reveal that the grains in the Ni-doped samples exhibit more facets than in the undoped samples. It is believed that the Sn ions stabilize the interface between the Zr-rich and Ti-rich domains, which appear during the cation-ordering transformation during sintering treatment [12]. The substitution of Sn for Zr in ZrTiO4 leads to a gradual decrease in the length scale of cation correlation induced by slow cooling. All these structural variations result in the modification of the spatial charge, which contributes to the microwave dielectric parameters: the dielectric constant εr , the dielectric loss tan δ, the quality factor Q, and the product Qf, where f is the working frequency. The NiO 0.2 wt.% addition induces significant modifications of the microwave dielectric parameters. The EDX data confirm the presence of a thin (Zn, Ni)2 TiO4 layer on grain surfaces, inhibiting the Sn ions segregation on the grain boundaries [7]. It can be considered that this effect induces changes in the spatial charge and consequently reduces the Sn contribution to the dielectric loss.
The EDX data show the Zr:Sn:Ti weight ratio of 58:12:30 for sample 4 of Table 3, and the ratio of 50:17:33 for sample 9. Therefore, the composition ratio in the presence of Ni ions is closer to the standard initial ratio 47:15:38 [7]. The experimental data listed in Table 3 show a decrease of the ZST dielectric loss in microwaves in the presence of Ni ions. The high values of Qf ranging from 32,000 to 58,000 indicate the Ni role to enhance the Sn capability to stabilize the interface between Zr-rich and Ti-rich domains, which appear during the cation-ordering transformations during the sintering treatment as revealed by other studies [12]. The effect of NiO is also a reduction of ZST sintering temperature, re-
Fig. 7. SEM image of ZST fracture. (a) ZST without NiO; (b) ZST with NiO.
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Table 4 Microwave dielectric parameters of ZST samples with NiO sintered at sintering temperatures 1280 and 1290 ◦ C Sample
Ts (◦ C)
Sintering time tp (h)
Resonance frequency fres (GHz)
Dielectric constant εr
Dielectric loss tan δ (×104 )
Quality factor Q
Product Qf (GHz)
10 11
1290 1280
2 2
7.2 9.24
36.3 35.9
2.2 1.63
4550 6123
32,760 56,576
Table 5 Quality factor measurements on a ZST sample in the millimeter-waves domain Frequency f (GHz)
Quality factor (Q)
Product Qf (GHz)
40.64 62.86 87.07 88.08 115.60
1300 900 600 600 440
52,832 56,574 52,242 52,848 50,864
The investigations in the millimeter-wave range showed the decrease of the quality factor from Q = 1300 at 40.64 GHz to Q = 440 at 115.6 GHz. The high dielectric constant, low dielectric loss, and controllable temperature coefficient recommend the ZST materials for microwave and millimeter-wave applications.
References sulting in a high quality ceramic material (low dielectric loss and high dielectric constant for this material) as shown in Table 4. The measurements shown a temperature coefficient τ f in the range −2 to +4 ppm/◦ C for NiO doped samples. The ZST samples sintered at 1290 ◦ C have larger dimensions, resonate at lower frequencies, and exhibit higher dielectric loss than samples sintered at 1280 ◦ C. The measured values of the quality factor Q are listed in Table 5 at frequencies up to 115.6 GHz for ZST disks sintered for 2 h at 1330 ◦ C with 0.2 wt.% NiO addition. The measurements revealed low dielectric loss for the ZST samples in the millimeter-wave domain.
4. Conclusions The investigations on (Zr0.8 , Sn0.2 )TiO4 revealed that the increase of the sintering temperature does not affect the dielectric constant essentially, but it results in the decrease of the dielectric loss. Moreover, the dielectric loss is further decreased by the sintering addition (0.2 wt.%) of NiO, while the other dielectric parameters, such as the dielectric constant εr and the temperature coefficient τ f , do not show significant changes.
[1] S.J. Fiedziuszko, I.C. Hunter, T. Itoh, Y. Kaobayashi, T. Nishikawa, S.N. Stitzer, K. Wakino, IEEE Trans. Microw. Theory Technol. MTT50 (2002) 706. [2] Y. Higuchi, H. Tamura, J. Eur. Ceram. Soc. 23 (2003) 2683. [3] R.J. Cava, J. Mater. Chem. 11 (2001) 54–62. [4] E.Y. Yuksei, T. Nagode, T.T.Y. Wong, IEE Proc. Microw. Antennas Propag. 151 (3) (2004) 221–226. [5] Y.H. Park, J.M. Ryu, M.Y. Shin, K.H. Ko, J. Am. Ceram. Soc. 84 (2001) 2542–2546. [6] M. Mizan, T. Higgis, D. Sturzbecher, in: Proceedings of the IEEE International Frequency Control Symposium, 1993, pp. 733–739. [7] F. Vasiliu, D. Grozea, C. Bunescu, S. Moisa, J. Mater. Sci. 29 (1994) 3337. [8] B.W. Hakki, P.D. Coleman, IRE Trans. on Microwave Theory and Techniques, MTT-8 (1960) 402–410. [9] G. Annino, M. Cassetari, I. Longo, M. Martinelli, IEEE Trans. on Microwave Theory and Techniques MTT-45 (1997) 2025. [10] G. Annino, D. Bertolini, M. Cassettari, M. Fittipaldi, I. Longo, M. Martinelli, J. Chem. Phys. 112 (2000) 2308. [11] K. Wakino, K. Minai, H. Tamura, J. Am. Ceram. Soc. 67 (1984) 278. [12] R. Christoffersen, P.K. Davies, X. Wei, J. Am. Ceram. Soc. 77 (1994) 1441. [13] H. Tamura, Am. Ceram. Soc. Bull. 73 (1994) 92. [14] American Standards for Testing and Materials, ASTM file, fiche no. 81–2214. [15] A. Ioachim, M.G. Banciu, M.I. Toacsan, G. Stoica, J. Electric. Eng. 53 (9/S) (2003) 160–162.
Nickel-doped (Zr0.8, Sn0.2)TiO4 for microwave and millimeter-wave applications A. Ioachima , M.G. Banciua , M.I. Toacsana , L. Nedelcua , D. Ghetua , H.V. Alexandrub,∗ , G. Stoicac , G. Anninod , M. Cassettarid , M. Martinellid a
National Institute of Materials Physics, Bucharest-Magurele, Romania b Faculty of Physics, University of Bucharest, Romania c S.C. IPEE S.A., Curtea de Arges, Romania d I.P.C.F.-C.N.R., Pisa, Italy
Abstract (Zr0.8 , Sn0.2 )TiO4 ternary compounds (ZST) have been prepared by conventional solid-state reaction from raw materials. The effects of such sintering parameters as sintering temperature, sintering time, and NiO addition on structural and dielectric properties were investigated. The material exhibits a dielectric constant εr ∼36.0 and high values of the product Qf of the intrinsic quality factor Q and the frequency f from 32,170 to 50,000 at microwave frequencies. The dielectric loss tan δ values of ZST ceramics are decreased by low-level doping of NiO, while the temperature coefficient of the resonance frequency τ f takes values in the range −2 to +4 ppm/◦ C. Investigations on whispering gallery modes revealed low dielectric loss in millimetre-wave domain. An intrinsic quality factor of 480 was measured at 115.6 GHz. Dielectric resonators and substrates of ZST material were manufactured. The dielectric properties make the ZST material very attractive to microwave and millimeter-wave applications, such as dielectric resonators, filters, planar antennas, hybrid microwave integrated circuits, etc. © 2005 Elsevier B.V. All rights reserved. Keywords: High dielectric constant; Dielectric resonators; Microwaves; Millimeter-waves
1. Introduction The development of modern microwave and millimeterwave communication systems requires new materials with appropriate properties [1] in specific frequency ranges. For many applications, dielectric materials with low loss and high dielectric constant [2,3] offer a high degree of miniaturization and improved performances [4]. Dielectric materials based on the ZrO2 –SnO2 –TiO2 ternary system [5] are very attractive for applications in the microwave domain due to their high dielectric constant, low loss, and controlled temperature coefficient of the permittivity [1,3]. Such compounds as (Zr0.8 , Sn0.2 )TiO4 exhibit an almost zero temperature coefficient and offer a great temperature stability to the specific applications such as frequency ∗ Corresponding author. Present address: University of Bucharest, Faculty of Physics, P.O. Box 74-165, Bucharest, Romania. Tel.: +40 21 413 3367; fax: +40 21 413 3367. E-mail address: [email protected] (H.V. Alexandru).
0921-5107/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2004.12.071
discriminators [4], low phase-noise dielectric resonator oscillators (DRO) [6], duplexers, and filters [1]. These materials are generally difficult to sinter without additives, especially at lower temperatures. The sinterability increases with additions, such as La2 O3 , ZnO, NiO, but no mechanisms have been proposed for the improved densification kinetics or for the addition effect on the dielectric loss. The La2 O3 addition has been reported as an improving sintering factor, which promotes the grain growth [7].
2. Experimental The (Zr0.8 , Sn0.2 )TiO4 ceramic materials were prepared by standard solid-state reaction technique. Powder oxides ZrO2 , SnO2 , and TiO2 (equivalent to a weight ratio of 47:15:38) with purity higher than 99.9% were mixed according to the (Zr0.8 , Sn0.2 )TiO4 stoichiometry. In order to reduce the sintering temperature, 2 wt.% La2 O3 and 1 wt.% ZnO were added.
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The powders were milled in the distilled water for 24 h in a mill with agate balls. All mixtures were dried and treated at 1200 ◦ C for 2 h. The calcined powders were then milled again for 2 h. For some samples, 0.2 wt.% NiO was added to the calcined powders, in order to investigate the effect on sintering properties of ZST. Pellets of 10 and 5 mm diameter were formed by uniaxial pressing and then sintered at 1280–1400 ◦ C for 2–4 h. The morphology, phase-composition, and microstructure of the sintered ceramics were analyzed by using scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) microanalysis. The crystalline phases were identified by X-ray diffraction (XRD) patterns. The bulk densities (ρr ) of the sintered pellets were measured by the Archimedes’ method. The structure of ZST was investigated by X-ray diffraction, Seifert Debye Flex 2002 diffractometer, provided with copper target X-ray tube (λ (Cu K␣ ) = 0.1541 nm). At microwaves, the ZST cylindrical samples exhibit very low dielectric loss, high dielectric constant, and a very good stability with temperature. Therefore, the dielectric parameters of ZST samples were investigated by using the Hakki–Coleman method [8]. A screen plot of the signal attenuation versus frequency is shown in Fig. 1 for a ZST sample with 5.94 mm height, 9.05 mm diameter, and εr = 36.6 dielectric constant. Each peak in Fig. 1 corresponds to a transversal electric, TE0np transversal magnetic TM0np or a hybrid electromagnetic HEMmnp resonance mode [1] where m, n, and p are the azimuthal, radial, and axial indices, respectively. The cursor C indicates the TE011 resonance mode, which is used in the Hakki–Coleman method [8]. A computer-aided measurement system combining a HP 8757C network analyzer and a HP 8350B sweep oscillator was employed in the microwave measurements. The temperature coefficient τ f of the resonant frequency in microwave range was measured by heating the samples from +18 to +90 ◦ C. The dielectric loss in millimetre-wave range was examined by investigating the whispering gallery modes (WGM) [9,10] of the samples with diameters between 5 and 10 mm and height between 2 and 4 mm. These modes were excited by
Fig. 1. The plot of the HP 8757C network analyzer screen when measuring a ZST cylindrical sample with 5.94 mm height, 9.05 mm diameter, and 36.6 dielectric constant. The attenuation vs. frequency curve exhibits resonance peaks corresponding to the resonant modes. The cursor C indicates the TE001 mode.
using a dielectric waveguide as shown in Fig. 2. The whispering gallery modes (WGMs) exhibit a strong confining of the electromagnetic field into the dielectric disk. Therefore, the WGMs allowed dielectric measurements in the millimeterwave range due to their very low radiation loss. An example of the WGMs resonating in a disk ZST is illustrated in Fig. 3. Each resonance dip shown in Fig. 3 corresponds to a WGM.
3. Results and discussions The effect of the sintering temperature Ts on ZST samples was analyzed. The investigated samples were generally multiphase. The X-ray diffraction patterns, which are presented in Figs. 4 and 5, showed that the (Zr0.8 , Sn0.2 )TiO4 compound was the majority phase, corresponding to the standard crystalline data [14]. At lower sintering temperatures, e.g. Ts = 1330 ◦ C, the minority phases, such as ZrTiO4 , and a
Fig. 2. Experimental setup for ZST disk characterization in the millimetre-wave range.
A. Ioachim et al. / Materials Science and Engineering B 118 (2005) 205–209
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Fig. 5. XRD pattern of ZST samples without NiO sintered at 1360 ◦ C/4 h.
Fig. 3. Amplitude of the reflected signal in the millimeter-wave range vs. frequency, showing whispering gallery modes (WGM) for dielectric loss measurements.
very small amount of unreacted TiO2 , were also present. For sintering temperatures Ts equal to or higher than 1360 ◦ C, the ZrTiO4 phase and TiO2 tend to disappear, and the crystalline lattice become more ordered. The X-ray diffraction peaks of (Zr0.8 , Sn0.2 )TiO4 are presented in Figs. 4 and 5 for samples sintered for 4 h at 1330 and 1360 ◦ C, respectively. It can be noticed that the increase of the sintering temperature Ts leads to higher and narrower XRD peaks, thus, to larger crystallites and a better-formed crystalline lattice. The XRD peaks do not change their positions when Ts is changed. Furthermore, the XRD peaks, which are not indexed in Figs. 4 and 5 and which correspond to the minority phases, decrease in intensity with the increase of the sintering temperature. This
effect shows an improved reactivity of the oxides at higher sintering temperature (Fig. 5). The increase of the grain sizes with the sintering temperature increase was confirmed by the SEM observations as shown in Fig. 6. When Ts increases, the grains become more faceted. For sintering temperatures Ts higher than 1400 ◦ C, some intergranular pores with polyhedral facets appear. They can be attributed to the SnO2 segregation at grain boundaries, followed by vaporization at higher temperatures. At temperatures greater than 980 ◦ C, SnO2 decomposes in metallic Sn and O2 . The metallic Sn has the tendency to diffuse out of the grains and to form white-colour spherules during the furnace cooling-down to room temperature [15]. The dependence of the unit-cell volume V0 and the EDX compositional data of the ZST ceramics on sintering temperature Ts is presented in Table 1. Samples 1 and 2 in Table 1
Fig. 6. SEM image of ZST ceramics. (a) Ts = 1330 ◦ C/4 h; (b) Ts = 1360 ◦ C/4 h.
Fig. 4. XRD pattern of ZST samples without NiO sintered at 1330 ◦ C/4 h.
Table 1 The unit-cell volume V0 and EDX compositional data of the (Zr0.8 , Sn0.2 )TiO4 ceramics in relation with the sintering temperature Ts Sample
Sintering temperature Ts (◦ C)
Sintering time tp (h)
Zr (wt.%)
Sn (wt.%)
Ti (wt.%)
a0 (nm)
b0 (nm)
c0 (nm)
V0 (nm3 )
1 2 3
1330 1360 1400
4 4 4
54.62 54.22 45.50
15.50 14.83 16.50
29.89 30.90 36.40
0.476 0.470 0.457
0.550 0.552 0.554
0.502 0.501 0.505
0.1315 0.1301 0.1279
a0 , b0 , and c0 are the unit-cell parameters and V0 is the unit-cell volume; wt.%: percent weight concentration of the element.
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Table 2 Dependence of microwave dielectric parameters on the sintering temperature Ts Sample
Sintering temperature Ts (◦ C)
Sintering time tp (h)
Resonance frequency f (GHz)
Dielectric constant εr
Dielectric loss tan δ (×104 )
Quality factor Q
Product Qf (GHz)
1 2 3
1330 1360 1400
4 4 4
6.09 6.03 6.78
35.30 33.90 35.34
5.5 3.7 1.9
2000 3000 5200
12,180 18,100 35,260
Table 3 Dependence microwave dielectric parameters on the NiO content and sintering time for the samples sintered at 1330 ◦ C Sample
NiO (wt.%)
Sintering time tp (h)
Bulk density ρ (g/cm3 )
Resonance frequency fres (GHz)
Dielectric constant εr
Dielectric loss tan δ (×104 )
Quality factor Q
Product Qf (GHz)
4 5 6 7 8 9
0 0 0 0.2 0.2 0.2
2.0 2.25 2.50 2.0 2.25 2.50
5.02 5.05 5.00 5.03 5.01 4.995
6.653 6.624 6.690 6.740 6.710 6.820
36.8 36.6 36.17 36.33 36.8 36.07
2.14 2.91 2.62 1.45 1.32 2.12
4672 3436 3816 6896 7575 4716
31,080 22,760 25,530 46,480 50,830 32,160
are rich in Zr, compared to the initial weight composition ratio. For sintering temperatures Ts ≥ 1400 ◦ C, the XRD patterns indicate the disappearance of the unreacted TiO2 [11]. The EDX data presented in Table 1 show the decrease of the volume V0 of the unit cell with the increase of the sintering temperature Ts . The Sn ions substitute the Zr ions or Ti ions to a certain degree. The decrease in the unit-cell volume V0 , while the sintering temperature Ts increases, suggests the preferential replacement of the Zr ions by the smaller Sn ions. This fact corresponds to a better short-range ordered structure [12]. The product Qf of the intrinsic quality factor Q and the frequency f of the samples with additions of La and Zn oxides, but without NiO, increases considerably with the sintering temperature Ts as shown in Table 2. The effect of sintering NiO addition on ZST characteristics was also investigated. The Ni ions, like Zn ions, do not diffuse into the grains, but remain at the boundary phase and form a spinel structure (Zn, Ni)2 TiO4 [13]. The SEM images in Fig. 7 reveal that the grains in the Ni-doped samples exhibit more facets than in the undoped samples. It is believed that the Sn ions stabilize the interface between the Zr-rich and Ti-rich domains, which appear during the cation-ordering transformation during sintering treatment [12]. The substitution of Sn for Zr in ZrTiO4 leads to a gradual decrease in the length scale of cation correlation induced by slow cooling. All these structural variations result in the modification of the spatial charge, which contributes to the microwave dielectric parameters: the dielectric constant εr , the dielectric loss tan δ, the quality factor Q, and the product Qf, where f is the working frequency. The NiO 0.2 wt.% addition induces significant modifications of the microwave dielectric parameters. The EDX data confirm the presence of a thin (Zn, Ni)2 TiO4 layer on grain surfaces, inhibiting the Sn ions segregation on the grain boundaries [7]. It can be considered that this effect induces changes in the spatial charge and consequently reduces the Sn contribution to the dielectric loss.
The EDX data show the Zr:Sn:Ti weight ratio of 58:12:30 for sample 4 of Table 3, and the ratio of 50:17:33 for sample 9. Therefore, the composition ratio in the presence of Ni ions is closer to the standard initial ratio 47:15:38 [7]. The experimental data listed in Table 3 show a decrease of the ZST dielectric loss in microwaves in the presence of Ni ions. The high values of Qf ranging from 32,000 to 58,000 indicate the Ni role to enhance the Sn capability to stabilize the interface between Zr-rich and Ti-rich domains, which appear during the cation-ordering transformations during the sintering treatment as revealed by other studies [12]. The effect of NiO is also a reduction of ZST sintering temperature, re-
Fig. 7. SEM image of ZST fracture. (a) ZST without NiO; (b) ZST with NiO.
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Table 4 Microwave dielectric parameters of ZST samples with NiO sintered at sintering temperatures 1280 and 1290 ◦ C Sample
Ts (◦ C)
Sintering time tp (h)
Resonance frequency fres (GHz)
Dielectric constant εr
Dielectric loss tan δ (×104 )
Quality factor Q
Product Qf (GHz)
10 11
1290 1280
2 2
7.2 9.24
36.3 35.9
2.2 1.63
4550 6123
32,760 56,576
Table 5 Quality factor measurements on a ZST sample in the millimeter-waves domain Frequency f (GHz)
Quality factor (Q)
Product Qf (GHz)
40.64 62.86 87.07 88.08 115.60
1300 900 600 600 440
52,832 56,574 52,242 52,848 50,864
The investigations in the millimeter-wave range showed the decrease of the quality factor from Q = 1300 at 40.64 GHz to Q = 440 at 115.6 GHz. The high dielectric constant, low dielectric loss, and controllable temperature coefficient recommend the ZST materials for microwave and millimeter-wave applications.
References sulting in a high quality ceramic material (low dielectric loss and high dielectric constant for this material) as shown in Table 4. The measurements shown a temperature coefficient τ f in the range −2 to +4 ppm/◦ C for NiO doped samples. The ZST samples sintered at 1290 ◦ C have larger dimensions, resonate at lower frequencies, and exhibit higher dielectric loss than samples sintered at 1280 ◦ C. The measured values of the quality factor Q are listed in Table 5 at frequencies up to 115.6 GHz for ZST disks sintered for 2 h at 1330 ◦ C with 0.2 wt.% NiO addition. The measurements revealed low dielectric loss for the ZST samples in the millimeter-wave domain.
4. Conclusions The investigations on (Zr0.8 , Sn0.2 )TiO4 revealed that the increase of the sintering temperature does not affect the dielectric constant essentially, but it results in the decrease of the dielectric loss. Moreover, the dielectric loss is further decreased by the sintering addition (0.2 wt.%) of NiO, while the other dielectric parameters, such as the dielectric constant εr and the temperature coefficient τ f , do not show significant changes.
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