Microstructure and Microwave Dielectric Properties of Modified Zinc Titanates (II)

MICROSTRUCTURE AND MICROWAVE DIELECTRIC PROPERTIES OFMODIFIED ZINC TITANATES (II)

H.T. Kim 1,2*, J.D. Byun1, and Y. Kim2

1Department of Materials Science and Engineering, Korea University,136–701 Seoul, Korea

2Division of Ceramics, Korea Institute of Science and Technology, 136–791 Seoul, Korea

(Refereed)(Received September 17, 1997; Accepted October 20, 1997)

ABSTRACTZinc titanates ZnO–TiO2 (Zn/Ti 5 0.67–2.0) and Zn12xMgxTiO3 (x 5 0–0.4)were prepared by a conventional mixed-oxide method, and the microstructureand dielectric properties in the microwave range were investigated. In theZnO–TiO2 system, zinc orthotitanate, Zn2TiO4, had rutile solubility up to 0.33mol followed by a decrease in the cubic lattice parameters as the amount ofrutile increased. The zerotf was obtained at near Zn/Ti5 1.15 composition.The er and Q*f of the composition were 25 and 23,000, respectively. In theZn12xMgxTiO3 system, the composite structure with zinc orthotitanate andrutile of 1ZnOz1TiO2 transformed into the (Zn, Mg)TiO3 hexagonal solidsolution at x5 0.3–0.4. However, phase decomposition occurred in this rangeat a temperature above 1160°C, which induced microcracks and resulted in adecrease in Q factors. A range of dielectric resonators wither 5 20–30, Q52,500–13,000 at 10 GHz, andtf 5 270 to 150 ppm/°C can be obtained inthis system at a sintering temperature as low as 1100°C.© 1998 Elsevier Science Ltd

KEYWORDS: A. oxides, B. chemical synthesis, C. X-ray diffraction, D.microstructure, D. dielectric properties

*To whom correspondence should be addressed.

Materials Research Bulletin, Vol. 33, No. 6, pp. 975–986, 1998Copyright © 1998 Elsevier Science LtdPrinted in the USA. All rights reserved

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975

INTRODUCTION

The demand for the development of dielectric materials for microwave frequencies isincreasing with the rapid progress in mobile and satellite communication systems. Highperformance with low loss and stable temperature coefficient of resonance frequency (tf) isthe basic requirement of dielectric resonators and filters.

Many materials have been developed and modified for specific applications [1,2]. Thedevelopment of good material systems is still in high demand. Zinc titanate (ZnO–TiO2) hasbeen used as a catalyst and pigment [3,4]. As a dielectric material, it was used up to now onlyindirectly as a component of solid solution in dielectric compositions [5]. The dielectricproperties of pure zinc titanate ceramics were investigated by Sugiura et al. [6]. Theymeasured low frequency (kHz) dielectric constant and foundte 5 0 for the compositioncontaining 53 mol% ZnO. Later, a study on low and microwave range was carried out byHaga et al. [7] for (12 x)ZnTiO3–xTiO2 systems, aiming to find a zerote andtf compo-sition. However, they succeeded only in a zerote composition, probably because of thecomplicated phase transformation of ZnTiO3, specifically the decomposition of ZnTiO3 intoZn2TiO4 and rutile at a temperature above 945°C [8].

In this experiment, new microwave dielectrics featuring temperature-compensating char-acteristics and a low sintering temperature with good microwave dielectric properties basedon ZnO–TiO2 and Zn12xMgxTiO3 (x 5 0–0.4) systems were prepared and the microstruc-ture and dielectric properties of the system in the microwave range were studied.

FIG. 1XRD patterns of ZnO–TiO2 system sintered at 1150°C for 4 h.

976 H.T. KIM et al. Vol. 33, No. 6

EXPERIMENTAL

The ZnO–TiO2 (Zn/Ti 5 0.67–2.0) and Zn12xMgxTiO3, x 5 0–0.40 compositions wereprepared by a conventional mixed-oxide method using ZnO (99.9%), MgO (99.9%), andTiO2 (99.9%). The starting materials were mixed by ball milling for 12 h and dried at 120°C.The mixed powders were calcined at 1000°C for 2 h and milled for 24 h. The ground powderwas pressed into discs 10 mm in diameter and 4.8 mm thick. Pellets were sintered at1100–1300°C for 4 h in air. The phase identification, microstructure, and compositionalanalysis of the sintered specimens were analyzed by XRD, optical microscopy, SEM, andEPMA-EDS. The microwave dielectric properties were measured using a HP-8720C networkanalyzer in s21 transmission mode. The unloaded Q factors at microwave frequencies weremeasured by the transmission open cavity method using copper cavity [9]. The dielectricconstants were measured by the Hakki-Coleman method with silver plate and calculated fromthe TE01d resonance mode value [10]. Thetf was measured by the open cavity method usingan invar cavity in a Heraeus chamber at220 to 170°C. The specimens were lapped andpolished with an aspect ratio of 0.45 for dielectric resonators.

RESULTS AND DISCUSSION

ZnO–TiO2 System. Figure 1 shows the XRD patterns of the ZnO–TiO2 (Zn/Ti 5 0.67–2.0)specimens sintered at 1150°C for 4 h. It was found that the zinc orthotitanate Zn2TiO4 hasa rutile solubility up to 0.33 mole. In this solubility region, a single phase ofa-Zn2TiO4

(JCPDS card 25–1164 (1980)) was obtained. With the excess rutile above 0.33 mole, thecomposite phase with zinc orthotitanate and uncombined rutile was observed and it agreedwith the SEM result. The intensities of the rutile phase were proportional to the Zn/Ti ratio.In the Zn/Ti 5 2.0 specimen,a-Zn2TiO4 with cubic structure and a small amount of ZnOdiffraction peaks were observed. The details of the XRD results are described in Table 1. Thecubic lattice parameter of the zinc orthotitanate decreased as the amount of rutile solubilityincreased. The lattice parameter of 8.4602 Å fora-Zn2TiO4 on JCPDS card 25–1164 is not

TABLE 1XRD Result and Density of ZnO–TiO2 System Sintered

at 1150°C for 4 h

Nominalcomposition Zn/Ti

Identifiedphasesa

Lattice parameter(60.0005 Å)

Density(g/cm3)

2ZnOz1TiO2 2.00 Z2T1 Z 8.4723 5.097ZnOz4TiO2 1.75 Z2T 8.4697 5.093ZnOz2TiO2 1.50 Z2T 8.4597 5.024ZnOz3TiO2 1.33 Z2T1 R 8.4580 4.975ZnOz4TiO2 1.25 Z2T1 R 8.4535 4.907ZnOz6TiO2 1.17 Z2T1 R 8.4556 4.899ZnOz8TiO2 1.13 Z2T1 R 8.4558 4.871ZnOz1TiO2 1.00 Z2T1 R 8.4558 4.892ZnOz3TiO2 0.67 Z2T1 R 8.4618 4.75

aZ2T: Zn2TiO4; Z: ZnO; R: TiO2 (rutile).

977ZINC TITANATESVol. 33, No. 6

similar to that of the 2ZnOz1TiO2 nominal composition, but is similar to that of the3ZnOz2TiO2 composition. From this result, it can be concluded that the high-temperatureform of zinc orthotitanate is Zn3Ti2O7 when sufficient rutile is provided. Figure 2 shows themicrostructure of ZnO–TiO2 ceramics sintered at 1150°C for 4 h. As in the XRD results,2.00, Zn/Ti # 1.50 specimens are single-phase solid solution, while specimens with Zn/Ti#1.50 have free rutile phase. The grain growth of the system was inhibited as the amount offree rutile phase increased.

The dielectric constants (er), quality factors (Q*f), and temperature coefficients of resonantfrequencies (tf) of the ZnO–TiO2 (Zn/Ti 5 0.67–2.0) sintered at 1150°C for 4 h are plottedin Figure 3. Thetf values changed from positive to negative as the amount of rutiledecreased. A zerotf value was obtained at around Zn/Ti5 1.15 composition. The pure zincorthotitanate (Zn2TiO4) rarely has microwave properties such that theer, Q*f, andtf of thosespecimens can be measured. The best quality factor in this system was obtained at theZn/Ti 5 1.75 composition and the value decreased as the amount of rutile increased. Thedielectric constants increased linearly with rutile content.

(Zn, Mg)TiO 3 System. The XRD data show that pure ZnTiO3 in this sintering rangeconsists of two phases, zinc orthotitanate and rutile (Fig. 4). From the EPMA-EDS result,however, the chemical composition of the presumed zinc orthotitanate phase was not exactly

FIG. 2SEM photographs of ZnO–TiO2 system sintered at 1150°C for 4 h: Zn/Ti5 (a) 2.0, (b) 1.5,(c) 1.0, and (d) 0.67.


the same as Zn2TiO4, but more like Zn3Ti2O7. This result indicates that Zn2TiO4 has asolubility of TiO2 about 0.33 mole; i.e., Zn2Ti1.33O4.66 at a sintering temperature above1000°C. In this solubility range, the cubic phase was still maintained and a slight decreasein lattice parameter was observed. The two-phase structure continued until the concentrationof Mg increased to,0.1 mole, except for the 1000°C calcined powder. At x5 0.20, anotherhexagonal phase had developed and single phase of hexagonal solid solution was accom-plished at x5 0.30–0.40 compositions (Fig. 4a,b). The lattice parameters of the specimenwere a 5 0.4988 nm andc 5 1.3851 nm. The structure is similar to that of the low-temperature form of zinc metatitanate, ZnTiO3, which has a rhombohedral Bravais lattice

FIG. 3Microwave dielectric properties of ZnO–TiO2 system sintered at 1150°C for 4 h.


FIG. 4XRD patterns of (Zn12xMgx)TiO3 with sintering temperatures: (a) 1000°C, (b) 1150°, (c)1200°C, and (d) 1250°C for 4 h (F Zn2TiO4(Zn3Ti2O7), ‚ rutile, , (Zn, Mg)TiO3 ss,fZn2Ti3O8).


symmetry [10], while Zn12xMgxTiO3, x 5 0.30–0.40 has a hexagonal lattice symmetry. Forthe x 5 0.20–0.40, the specimens decomposed into three phases at a sintering temperatureabove 1160°C (Figs. 4c,d and 7). This decomposition induced microcracks in the sinteredcompacts. It seems that the (Zn, Mg)TiO3 hexagonal solid solutions are the low-temperaturestable form below 1160°C.

Typical SEM microstructures obtained for Zn12xMgxTiO3, x 5 0–0.40 ceramics sinteredat 1150°C for 4h are shown in Figure 5. No such noticeable microstructure change was

FIG. 5SEM photographs of the polished surface of (Zn12xMgx)TiO3 specimens sintered at 1150°Cfor 4 h: (a) x5 0, (b) x 5 0.05, (c) x5 0.10, (d) x5 0.20, (e) x5 0.30, and (f) x5 0.40.


observed at x5 0–0.10, but three phases were developed at around x5 0.20 specimen. Thesingle-phase solid solution microstructure was seen at x5 0.30–0.40, which agrees with theresult of XRD analysis. The EPMA-EDS results for the x5 0.20 specimen sintered at1150°C for 4 h are described in Figure 6 and Table 2.

From the EDS results, it was found that the specimen consists of cubic (Zn, Mg)2TiO4

phase (W1), hexagonal (Zn, Mg)TiO3 phase (G1), and rutile phase (B1) with small amountsof Mg and Zn incorporated. The optical microscopy of the microstructure evolution ofdecomposition in the x5 0.30 specimen is shown in Figure 7. The enlarged details by SEMsecondary and backscattered electron image and EPMA-EDS analysis results are shown inFigure 8 and Table 3. From the EDS results, it was found that the (Zn, Mg)TiO3 solid solutiondecomposed into three phases: cubic (Zn, Mg)2TiO4 phase (W2), hexagonal (Zn, Mg)TiO3

phase (G2), and cubic (Zn, Mg)2Ti3O8 phase (B2); these results corresponded well to thoseof XRD analysis.

FIG. 6Backscattered electron image of the (Zn0.8Mg0.2)TiO3 specimen sintered at 1150°C for 4 h.

TABLE 2EPMA-EDS Results of the (Zn0.8Mg0.2)TiO3 Specimens

Sintered at 1150°C for 4 h

Phase notation Element % Element6 error Atomic %

W1 Mg 3.26 0.5 7.2Zn 63.56 1.4 54.1Ti 33.36 0.4 38.7

G1 Mg 5.06 0.5 10.7Zn 49.96 1.2 40.0Ti 45.16 0.5 49.3

Bi Mg 0.6 6 0.3 1.2Zn 13.26 0.9 9.9Ti 86.26 0.7 88.9


Figure 9 shows the microwave dielectric properties of Zn12xMgxTiO3, x 5 0–0.40specimens sintered at 1100–1300°C for 4 h. The dielectric resonators were measured at 9–11GHz and the Q factors were normalized at 10 GHz. As the amount of Mg increased, thedielectric constant andtf decreased. The dielectric constant of pure ZnTiO3, which consistsof Zn2TiO4 solid solution and rutile, was about 30 and decreased to about 20 as the amountof x increased to 0.25. No noticeable decrease in dielectric constants was observed at x50.25–0.40.

FIG. 7Optical microscopy of the decomposition process of (Zn0.7Mg0.3)TiO3 specimen sintered at1150–1200°C for 4 h.


TABLE 3EPMA-EDS Results of the (Zn0.7Mg0.3)TiO3 Specimens

Sintered at 1200°C for 4 h

Phase notation Element % Element6 error Atomic %

W2 Mg 6.16 0.6 13.1Zn 56.16 1.4 45.1Ti 37.96 0.5 41.7

G2 Mg 9.36 0.6 18.4Zn 37.66 1.3 27.9Ti 53.16 0.6 53.7

B2 Mg 10.86 0.5 20.6Zn 26.16 1.2 18.5Ti 63.16 0.6 60.9

FIG. 8The enlarged SEM photograph of the (Zn0.7Mg0.3)TiO3 specimen sintered at 1200°C for 4 h(a) and its backscattered electron image (b).


FIG. 9Microwave dielectric properties of (Zn, Mg)TiO3 ceramics.


The tf also decreased from150 ppm/°C to270 ppm/°C due to the elimination of thepositivetf phase of residual rutile in pure ZnTiO3 through the formation of the negativetf

phase of (Zn, Mg)TiO3 solid solution, which has a MgTiO3-like hexagonal ilmenite structure.The zerotf was obtained at about x5 0.15.

The Q factors increased from 2,500 to 5,000–13,000. The increase in Q factors can beascribed to the crystal structure change from the low Q form of zinc orthotitanate with rutilecomposite phase to the high Q form of hexagonal lattice single phase. As in the precedingdiscussion, decomposition-induced microcracks occurred in the specimens of x5 0.20–0.40and resulted in the significant decrease in Q factors. Thetf and dielectric constant, however,were not influenced by the decomposition.

CONCLUSION

The microwave dielectric properties of newly developed zinc titanates (ZnO–TiO2, Zn/Ti 50.67–2.0) and zinc magnesium titanates (Zn12xMgxTiO3, x 5 0–0.4) systems were inves-tigated. The temperature coefficients of resonant frequencies in the systems depend on thephase contents. Especially, the amount of rutile had significant effects on the dielectricproperties of the system. The Q of the (Zn, Mg)TiO3 was degraded by decomposition, whichwas accompanied by microcracks, at a sintering temperature above 1160°C. Thetf and er

values, however, were not influenced by the decomposition.

ACKNOWLEDGMENT

The authors express their special thanks to Mr. Young Gil Yoon and Mr. Gi Hoon Park ofthe analytical center of the Korea Institute of Science and Technology and also thank Prof.Sahn Nahm for many helpful discussions.

REFERENCES

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Microstructure and Microwave Dielectric Properties of Modified Zinc Titanates (II)