Microstructures and microwave dielectric properties of Li2O-Nb2O5-ZrO2 ceramics

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Short communication

Microstructures and microwave dielectric properties

of Li

2

O–Nb

2

O

5

–ZrO

2

ceramics

Chun-An Lu

a

, Pang Lin

a

, Sea-Fue Wang

b,

*

a

Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu, Taiwan, ROC

b_{Department of Materials and Mineral Resources Engineering, National Taipei University of Technology, 1, Section 3,}

Chung-Hsiao East Road, Taipei, Taiwan, ROC

Received 27 November 2005; received in revised form 29 March 2006; accepted 29 April 2006 Available online 15 September 2006

Abstract

Solid solution of Li14yxZry+4xNb13xO3system is a new non-stoichiometric compound with orthorhombic perovskite structure. In this study, the effects of the calcination and the sintering temperatures on the microstructural evolution and microwave dielectric properties of Li14yxZry+4xNb13xO3ceramics were performed. Dense Li0.774Zr0.057NbO3ceramics can be obtained at the sintering temperature of 1150 8C. Li0.774Zr0.057NbO3phase exists as the main phase with the existence of a minor LiNb3O8phase. Typical microwave dielectric properties for dense Li0.774Zr0.057NbO3ceramics are as followed: er 39, Q f 4500, and tf=16.6 ppm/8C, measured at 6 GHz.

Keywords: Microwave properties; Densification; Microstructural evolution; Li2O–Nb2O3–ZrO2system

1. Introduction

The recent rapid expansion of telecommunication systems demands dielectric resonators (DRs) as basic components for designing filters and oscillators. Now, dielectric ceramics for use in resonators at microwave frequency have been paid increasing attention due to the fast growth of mobile communication systems such as cellular phone, global positioning systems and personal communication system. For the applications in microwave devices, a high dielectric constant (e > 20), a high dielectric loss quality (Q > 2000), and a near zero temperature coefficient of resonant frequency (0–10 ppm/8C) are required. High dielectric constant makes possible to reduce the size of the material by factor of 1/er1/2 so that size of the circuit can be

reduced considerably. The high Q values enable low insertion loss and low bandwidth of the resonance frequency, which is required for achieving high frequency selectivity and stability in the microwave transmitter components.

Recently, dielectric material often required to be co-fired with high conductivity electrode such as Ag and Cu in order to minimize the microwave absorption loss or to form a multilayer

structure to increase the volume efficiency. However, the sintering temperatures of common dielectric ceramics are in the range between 1200 and 1500 8C, which is much higher than the melting temperature of Ag (961 8C) or Cu (1064 8C). For instance, the sintering temperatures of BaO–Nd2O3–TiO2–

Nb2O5, Ba6xLn8+2x/3Ti18O54 and (Zr,Sn)TiO4 systems are

around 1325, 1350 and 1400 8C, respectively [1–3]. There is considerable interest in the development of new materials with low sintering temperatures. One way involved is the investigations of the glass-forming additives on the properties of established microwave materials. For instance, the BaO–La2O3–

4.7TiO2ceramic with the addition of 20 wt.% PbO–B2O3–SiO2

aids can reduce the sintering temperature down to 900 8C, but the microwave properties were degraded[4]. Another way is the use of the new material systems with lower sintering temperatures [5]. For instance, the sintering temperature of Bi12MO20d

(M = Si, Ge, Ti, Pb, Mn, B1/2P1/2), TiO2–TeO2, Bi2O3–ZnO–

Nb2O5, and Li2O–Nb2O5–MO2 (M = Ti, Zr) are around 680–

850, 720, 950, and 1150 8C, respectively[6–8].

Among the low sintering compounds, Villafuerte-Castrejon et al. [9] were first to report the solid solutions of Li14yxZry+4xNb13xO3 system, for which y = 0.057 and

0 < x < 0.15 system. This new system is a non-stoichiometric compound with orthorhombic perovskite structure which is fairly close to LiNbO3. The structure is based on a complete

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* Corresponding author. Fax: +886 2 2731 7185. E-mail address:[email protected](S.-F. Wang).

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three-dimensional array of corner-sharing NbO6octahedra. Up

to now, there have been no reports on their microwave dielectric properties yet. In this study, the solid solution of Li0.774Zr0.057NbO3 (y = 0.057, x = 0) was used throughout.

The effects of the calcination and the sintering temperatures on the microstructural evolution and microwave dielectric proper-ties of Li0.774Zr0.057NbO3are investigated, and the relationships

among them are discussed. 2. Experimental procedure

Ceramic samples were prepared through the conventional solid-state reaction of reagent-grade Li2CO3 (99.9%), Nb2O5

(99.8%) and ZrO2(99%). The powders were mixed in methyl

alcohol solution using polyethylene jars and zirconia media for 8 h. After drying, the powders were preheated at 700 8C to remove carbon dioxide at 700 8C, and followed by calcining at 1000, 1050, 1100 and 1150 8C for 10 h. After calculation, the powders were remilled in methyl alcohol for 12 h. The powders have particle size (d50) of0.34 mm measured by light scattering

(Zeta 1000) and surface area of2.3 m2/g from B.E.T. method. The granulated powders with a PVA binder were then dry-pressed under 1 ton to form pellets (9 mm in diameter). They were then de-bindered at 550 8C for 4 h and subsequently sintered in air at 950, 1000, 1050, 1100 and 1150 8C for 1 h, with a heating rate of 10 8C/min. Differential thermal analysis (DTA) was performed in a Pt crucible with a heating rate of 10 8C/min using a Perkin-Elmer Calorimeter (Series 1700 DTA), on mixtures to evaluate the melting reactions.

The densified cylindrical samples were polished to have an exact thickness of 5 mm for the measurements of microwave properties. Bulk densities of the sintered samples were measured using the Archimedes method with de-ionized water. Phase identification on the calcined powders as well as the sintered bulk ceramics were performed using X-ray diffraction (XRD, Simens D5000). The microstructures of the sintered samples were observed using scanning electron microscopy (SEM, JEOL 6500F). The grain size was

determined from SEM micrographs by linear intercept method. The sintered samples were further inspected by transmission electron microscopy (Model Tecnai 20 by Philips), to understand the distribution of various species. The dielectric constant (er) and quality factor (Q f) were

evaluated, based on the cylindrical cavity method (cavity 1005CIRC and software CAVITY, Damaskos Inc.), using a HP 8722D network analyzer. Detailed measurement procedures have been described elsewhere [10]. The temperature coefficient of resonant frequency (tf) was measured

within the range from 25 to 80 8C. The tf was defined by

ð fT f25Þ= f25ðT 25CÞ in Damaskos cavity.

3. Results and discussion

XRD patterns for the Li0.774Zr0.057NbO3powders calcined at

different temperatures for 10 h are shown in Fig. 1. For the

Fig. 1. XRD patterns of Li0.774Zr0.057NbO3powders with different calcination

temperatures.

Fig. 2. Densities of the Li0.774Zr0.057NbO3 ceramics sintered at different

temperatures, for the powders prepared at different calcinations temperatures.

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powders calcined at 1000 and 1050 8C, LiNbO3and LiNb3O8

phases co-exist. Peaks attributed to ZrO2are not observed in the

XRD patterns due to its small quantity. As the calcination temperature reaches 1100 8C, Li0.774Zr0.057NbO3phase starts

to form. The peak intensities of LiNb3O8and LiNbO3phases

continue to decrease as the calcination temperature increases.

This is due to the formation of Li0.774Zr0.057NbO3 solid

solution that induces the LiNbO3phase to release Li content,

which then reacts with LiNb3O8 and ZrO2 to generate more

Li0.774Zr0.057NbO3.Well-crystallized Li0.774Zr0.057NbO3phase

with orthorhombic structure was obtained at 1150 8C accom-panied with minor LiNb3O8and LiNbO3phases.

The theoretical sintered densities of Li0.774Zr0.057NbO3

ceramics sintered at different temperatures are indicated in Fig. 2. The curves in the figure correspond to the powders previously calcined at 1050, 1100, and 1150 8C, respectively. The results show that over 95% theoretical densities can be obtained when the sintering temperature reaches 1150 8C, regardless the previous calcination temperatures. Sintering temperature beyond 1150 8C was not pursued due to the fact that a liquid phase formation at 1190 8C, according to the DTA result (Fig. 3). Fig. 4 gives the XRD patterns of Li0.774Zr0.057NbO3 ceramics prepared from the powders

calcined at different temperatures and subsequently sintered at 1150 8C. In all cases, Li0.774Zr0.057NbO3phase exists as the

main phase with the existence of a minor LiNb3O8phase, which

may result from the loss of Li by evaporation.

The SEM microstructures of ceramics prepared from various calcination and sintering temperatures are shown in Fig. 5. It seems that the microstructures are closely correlated to the Fig. 4. XRD patterns of Li0.774Zr0.057NbO3ceramics, with different calcination

temperatures, sintered at 1150 8C.

Fig. 5. SEM micrographs of Li0.774Zr0.057NbO3ceramics prepared from different calcination temperatures (CT) and different sintered temperatures (ST): (a)

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sintering temperature but not the calcination temperature. Generally, the ceramics of Li0.774Zr0.057NbO3 were porous

when sintered at 1050 8C. As the sintering temperature increased to 1150 8C, dense Li0.774Zr0.057NbO3ceramics were

obtained accompanied with a significant grain growth.Fig. 6 shows the average grain size of Li0.774Zr0.057NbO3ceramics at

different sintering temperatures. The grain sizes of Li0.774Zr0.057NbO3 ceramics increase gradually from 2 to

4 mm as the sintering temperature increased from 1000 to 1100 8C, in spite of the different calcination temperatures. Significant grain growth was observed for sintering temperature above 1150 8C, at which the grain sizes are in the range of 5– 9 mm.

In order to reveal the detailed information of the structure, Li0.774Zr0.057NbO3 ceramic was examined by TEM and the

result is shown inFig. 7. The areas A and B in the bright field image had a similar diffraction pattern, but discrepancy in the d-space and the face angle. The electron diffraction patterns identified by d-space are corresponding to the Li0.774Zr0.057NbO3 phase. The area C was identified as

LiNb3O8phase. These results are coincidence with the XRD

results discussed above. In the bright field, it exhibited the growing lath crystal that is derived from the grain coarsing through the surface diffusion and boundary diffusion. Furthermore, it is found that the presence of the amorphous phase in the microstructure, as shown inFig. 7. The amorphous phase was not shown in the XRD results due to its small quantity, which may be caused by the evaporation and condensation of the Li compounds.

Table 1 shows the microwave dielectric properties of the typical Li0.774Zr0.057NbO3 ceramics prepared from various

calcination and sintering temperatures. The dielectric constants of dense Li0.774Zr0.057NbO3ceramics are ranging from 28 to 33.

It exhibits the same trend as the density, which increases with the sintering temperatures. The Q f values of Li0.774Zr0.057NbO3ceramics are ranging from 3000 to 4500,

which have a strong correlation with the grain size, as shown in Fig. 5. This is due to the facts that a larger grain size, derived

Fig. 7. Transmission electron micrographs of the Li0.774Zr0.057NbO3ceramics,

with calcination temperature at 1150 8C, sintered at 1150 8C: (a) TEM BF image of Li0.774Zr0.057NbO3ceramics after sintering showing the areas A and B

are Li0.774Zr0.057NbO3phase and area C is LiNb3O8, and (b) TEM BF image of

Li0.774Zr0.057NbO3ceramics with the existence of amorphous phase.

Table 1

Microwave dielectric properties of Li0.774Zr0.057NbO3 ceramics prepared at

different conditions Calcination temperature (8C) Sintering temperature (8C) er Q f (GHz) tf(ppm/8C) 1000 1150 33.01 4463.63 28.58 1050 1150 27.75 3371.90 24.79 1100 1150 38.66 2935.80 16.63 1150 1150 30.97 3551.38 15.91

Fig. 6. Grain sizes of Li0.774Zr0.057NbO3ceramics with different calcination

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from a higher sintering temperature, possesses a better ordering of the ions, which produces the higher Q f values.Table 1 also shows that tfvalue is strongly dependent on the calcination

temperature. Typically, a higher calcination temperature enabled the temperature coefficient of resonant frequency near zero.

4. Conclusion

The microstructure and the microwave dielectric properties of the new orthorhombic perovskite phase Li0.774Zr0.057NbO3

were investigated. High-density Li0.774Zr0.057NbO3 ceramics

(>95% T.D.) can be obtained after sintered at 1150 8C. XRD results show that Li0.774Zr0.057NbO3phase exists as the main

phase with a minor LiNb3O8phase, which is confirmed by TEM

results. The grain sizes of ceramics sintered at 1150 8C are in the range of 5–9 mm. Typical microwave dielectric properties for dense Li0.774Zr0.057NbO3ceramics are as followed: er 39,

Q f 4500, and tf=16.6 ppm/8C, measured at 6 GHz.

The dielectric constants and Q f values of Li0.774Zr0.057NbO3

ceramics have a strong correlation with the density and grain size, respectively.

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