Effects of additives on microstructures and microwave dielectric properties of (Zr, Sn)TiO4 ceramics

Materials Chemistry and Physics 71 (2001) 17–22

Effects of additives on microstructures and microwave dielectric properties of (Zr, Sn)TiO4 ceramics Cheng-Liang Huang a,∗ , Min-Hung Weng a , Hui-Liang Chen b a

Department of Electrical Engineering, National Cheng Kung University, 1 University Road, Tainan, Taiwan, ROC b Tainan Woman’s College of Art and Technology, Tainan, Taiwan, ROC Received 8 March 2000; received in revised form 28 June 2000; accepted 20 October 2000

Abstract The effects of CuO, Bi2 O3 and V2 O5 additives (up to 3 wt.%) on the microstructures and the microwave dielectric properties of (Zr0.8 Sn0.2 )TiO4 ceramics are investigated. The sintering temperature of (Zr0.8 Sn0.2 )TiO4 ceramics with CuO, Bi2 O3 and V2 O5 additions can be effectively reduced from 1400 to 1300◦ C due to the liquid phase effects. The grain grows at lower sintering temperatures (1300◦ C) to 4.5–5.4 ␮m with CuO, Bi2 O3 or V2 O5 additions. The dielectric constants (ε r ) are not significantly affected by various additives and ranged in 36–38. Small values (<±3 ppm ◦ C−1 ) of the temperature coefficient of resonant frequency (τ f ) are obtained for (Zr0.8 Sn0.2 )TiO4 ceramics with additives. However, the unloaded quality factors Q × f are strongly dependent upon the type and the amount of additives. © 2001 Elsevier Science B.V. All rights reserved. Keywords: ZST ceramics; Liquid phase sintering; Dielectric properties

1. Introduction Commercial wireless technologies, such as cellular phones, direct broadcasting satellite (DBS) and global positioning systems, have been making rapid progress due to the improved performance of dielectric resonators at microwave frequencies. Requirements for these dielectric resonators must be combined with a high dielectric constant (ε r > 20) for possible size miniaturization (size of a dielec√ tric resonator is −1/ εr ), a low dielectric loss (Q > 5000, where Q = 1/tan δ) for high frequency selectivity and low signal attenuation, and a near-zero temperature coefficient of resonant frequency (τ f ) for temperature stable circuits [1]. Dielectric materials with lower dielectric constants often exhibit higher unloaded quality factors. Table 1 illustrates some commercial dielectric resonator materials [2–4]. A solid solution of ZrO2 , TiO2 and SnO2 , especially the composition Zr0.8 Sn0.2 TiO4 (defined as ZST), offers perhaps the best microwave dielectric properties and ease of processing for microwave components [5]. ZST ceramics are difficult to be densified without any sintering aid. With 1 wt.% ZnO addition, ZST ceramics sintered at 1400◦ C could give properties of εr ∼ 37.8, Q × f ∼ 45,000 and τf ∼ 0 ppm ◦ C−1 . Many papers reported the microstructures ∗ Corresponding author. Tel.: +886-6-2757575; fax: +886-6-2345482. E-mail address: [email protected] (C.-L. Huang).

and the microwave dielectric properties of ZST ceramics with small amount of additives such as Fe, La, Ni, Co and Zn [6–9]. However, few works have been done in lowering the sintering temperature. The liquid-phase sintering by glass addition was found to effectively lower the firing temperature, while it also decreased the microwave dielectric properties of dielectric resonators. For example, Takada et al. [3] tried to sinter ZST ceramics with glass flux below 1200◦ C. The densities, in fact, were too low (<70% of theoretical densities, TDs). The ε r and Q × f values were also lower than 20 and 25,000, respectively. Another method used for lowering the sintering temperature was chemical processing. Kudesia et al. [8] prepared ZST ceramics with 0.05 wt.% La2 O3 and 1.0 wt.% ZnO using a co-precipitation technique and sintering at 1325◦ C for 16 h. The ceramics exhibited high relative density >99.1% and good dielectric properties: εr = 37.6, Q × f = 55,000 and τf = −2.9 ppm ◦ C−1 . However, the chemical process often required a flexible procedure that would increase the cost and time in fabricating the dielectric resonators. The melting temperatures of V2 O5 , Bi2 O3 and CuO are 650, 868 and 1326◦ C, respectively. In the past, liquid phase flux such as Bi2 O3 and CuO were introduced as sintering aids in lowering the sintering temperature of ceramics and obtaining good dielectric properties [10,11]. In this paper, V2 O5 , Bi2 O3 and CuO were added in ZST ceramics to lower the sintering temperature. Since the microwave dielectric properties of ZST ceramics were

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Table 1 Commercial ceramic materials for dielectric resonators Dielectric materials

STa (◦ C)

εr

Q × f (GHz)

τ f (ppm ◦ C−1 )

(Mg, Ca)TiO3 Zr0.8 Sn0.2 TiO4 BaO–TiO2 –WO3 (Ba, Sr)O–Sm2 O2 –TiO2

1400 1450 1400 1350

21 38 39 78

70000 45000 40000 6000

0 0 0 <10

a

ST, sintering temperature.

dependent upon the raw materials, the additives, and the microstructures, the investigation at low sintering temperature were conducted with different type and amount of additives.

2. Experimental procedure

τf =

f80 − f20 × 106 (ppm ◦ C−1 ) 60 × f20

(1)

where f20 and f80 are TE01δ resonant frequency at 20 and 80◦ C, respectively. A system including a HP8757D network analyzer and a HP8350B sweep oscillator was employed in the measurement.

2.1. Sample preparation Samples of (Zr0.8 , Sn0.2 )TiO4 were synthesized by conventional mixed oxide methods from individual highpurity oxide powders (>99.9%): ZrO2 , SnO2 and TiO2 . The starting materials were mixed in accordance with desired stoichiometry ZST ceramics with 1 wt.% ZnO as sintering aid. The powders were ground in distilled water for 6 h in a balling mill with agate balls. Mixtures were dried and calcined at 1100◦ C for 3 h. Different amount of V2 O5 , Bi2 O3 and CuO were individually added in the calcined powder and remilled for 5 h with PVA solution as a binder. Pellets with 11 mm diameter and 5 mm thick were pressed by uniaxial pressing. After debinding, these pellets were sintered at temperatures 1220–1340◦ C for 3 h. 2.2. Microstructure analysis The microstructure observation of sintered ceramic surface was performed by means of scanning electron microscopy (SEM, JOEL, JEL-6400) and energy dispersive spectroscopy (EDS). The crystalline phases of sintered ceramics were identified by X-ray diffraction pattern (XRD, Rigaku D/Max III.V). The bulk densities of the sintered pellets were measured by the Archimedes method. The grain sizes were evaluated using the linear intercept method. 2.3. Dielectric measurement The dielectric constants (ε r ) were calculated by the sizes of sample and the frequency of TE011 mode using the Hakki–Coleman dielectric resonator method [12]. The unloaded quality values Q at microwave frequencies were measured by improved resonant method after Kabayashi [13]. Since Q × f keeps constant in the microwave region, the unloaded quality factors were expressed as Q × f values. The temperature coefficient of resonant frequency (τ f ) at microwave frequency was measured in the temperature range from 20 to 80◦ C, and calculated by Eq. (1).

3. Results 3.1. Phase identification Fig. 1 shows the XRD patterns of ZST powders calcined at different temperatures for 3 h. The existing phases at each temperature were observed as illustrated in the figure. With the increase of calcined temperature, the mixed powders reacted more and the intensity of ZST phases enhanced. Homogeneous ZST phase with ␣-PbO orthorhombic structure was obtained at 1150◦ C for 3 h. The XRD patterns of ZST ceramics with CuO and Bi2 O3 additions sintered at 1300◦ C for 3 h were shown in Fig. 2. At the level of 0.5–2 wt.% CuO and Bi2 O3 additions, ZST ceramics exhibited single phase. However, second phase (marked as ‘∗’) was detected in the specimens with 3 wt.% Bi2 O3 addition. XRD patterns of ZST ceramics with 0.5–2 wt.% V2 O5 additions shown in Fig. 2 also demonstrated single phase as that with CuO addition case. 3.2. Densification and microstructure Fig. 3 showed the densities of ZST ceramics with different amount of additions sintered at 1300◦ C. The densities of ZST ceramics increased with increasing CuO and Bi2 O3 addition up to 1 and 2 wt.%, respectively, and then decreased. The decrease in density for 3 wt.% Bi2 O3 addition was considered as the presence of second phase. With V2 O5 addition, the densities decreased with increasing amount of V2 O5 content. The maximum densities of ZST ceramics were obtained at 1 wt.% CuO and 2 wt.% Bi2 O3 addition. Although V2 O5 has lower melting temperature than the other two additives, it seemed that CuO and Bi2 O3 could more effectively lower the sintering temperature. The densification at low sintering temperature was caused by the liquid phase effect of CuO addition as discussed latter. In general, the density of ceramics increased with increasing

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Fig. 1. XRD patterns of pure (Zr0.8 , Sn0.2 )TiO4 powders calcined at different temperature for 3 h.

sintering temperature and then decreased after reaching its maximum value. ZST ceramics with different additives were sintered at constant temperature for comparison. The sintering temperature 1300◦ C was selected since that all the samples had densities higher than 95% TD. The SEM photographs of ZST ceramics with different additives sintered at 1300◦ C for 3 h were illustrated in Fig. 4.

Fig. 2. XRD patterns of (Zr0.8 , Sn0.2 )TiO4 ceramics sintered at 1300◦ C for 3 h with: (a) 1 wt.%; (b) 2 wt.%; (c) 3 wt.% Bi2 O3 additions and with: (d) 0.5 wt.%; (e) 1 wt.%; (f) 2 wt.% CuO additions.

All samples showed homogeneous grain sizes except that with 3 wt.% Bi2 O3 addition. Liquid phase effect was observed in the grain morphology shown in Fig. 4. The liquid phase was believed caused by eutectic of CuO–Cu2 O–TiO2 (Cu3 TiO4 ) at 1070◦ C from phase diagram. In addition to liquid phase, second phase was not observed at the level of 0.5–2 wt.% V2 O5 and CuO. XRD showed that ZST ceramics with CuO addition exhibited single phase since that detection of a minor phase by X-ray is extremely difficult. In the

Fig. 3. The densities of (Zr0.8 , Sn0.2 )TiO4 ceramics with additions sintered at 1300◦ C as functions of the amount of additives.

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Fig. 4. SEM micrographs of (Zr0.8 , Sn0.2 )TiO4 ceramics with 1 wt.%: (a) CuO; (b) Bi2 O3 ; (c) V2 O5 and (d) 3 wt.% Bi2 O3 additions at 1300◦ C.

case of 3 wt.% Bi2 O3 addition, lath-shaped grains appeared as second phase rich in Bi detected by EDS. The grain sizes of ZST ceramics with different additives were illustrated in Fig. 5. Grain size of ZST ceramics with CuO was larger than those with Bi2 O3 and V2 O5 . It was believed that the grain wetting ability of CuO addition was better than that of Bi2 O3 and V2 O5 additions in ZST ceramics. Grain sizes of 4.5–5.4 ␮m were obtained in the experiment. The grain sizes slightly increased with the increase of Bi2 O3 or CuO (up to 1 wt.%) addition while remained nearly no change of the case of V2 O5 addition. 3.3. Dielectric properties Fig. 6 showed the dielectric constant of ZST ceramics with different additives. The dielectric constant presented

Fig. 5. The grain size of (Zr0.8 , Sn0.2 )TiO4 ceramics with additions sintered at 1300◦ C as functions of the amount of additives.

the same trend as the density in all the cases, and was in the range of 35–39. Fig. 7 illustrated the Q × f values of ZST ceramics with different additives. The Q × f values were varied with different type and amount of additives. In CuO case, the Q × f value increased from 28,000 (0.5 wt.%) to 45,000 (1 wt.%) and then decreased with further increase in the additive amount. The Q × f value of ZST ceramics with V2 O5 addition exhibited the same trend and reached its maximum value of 51,200 at 1 wt.% addition. However, the Q × f value rapidly decreased with increasing amount of Bi2 O3 . The Q × f value was less than 8000 for ZST with 3 wt.% Bi2 O3 addition. The physical and dielectric properties of ZST ceramics with different additives sintered at 1300◦ C for 3 h are summarized in Table 2. The temperature coefficient of resonant

Fig. 6. The dielectric constants of (Zr0.8 , Sn0.2 )TiO4 ceramics with additions sintered at 1300◦ C as functions of the amount of additives.

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XRD and SEM. It may be due to the fact that the significant difference in ionic size between the substitution ion and the ion in ZST. Since the ion radius of Bi3+ (0.96 Å) is much larger than that of Zr4+ (0.72 Å), Sn4+ (0.69 Å) and Ti4+ (0.605 Å) [13], Bi3+ ion could not be all substituted for tetravalent ions in ZST lattice. 4.2. Factors on dielectric constant and Q × f value

Fig. 7. The Q × f values of (Zr0.8 , Sn0.2 )TiO4 ceramics with additions sintered at 1300◦ C as functions of the amount of additives.

frequency (τ f ) was related to composition and second phase. The τ f values did not change much since that ZST ceramics was a well known temperature stable material and the additives did not cause other second phase except liquid phase. As addition level increased, the τ f value became more negative although the change was less than ±3 ppm ◦ C−1 . 4. Discussion 4.1. Effect of additives on sintering behavior and microstructure ZST ceramics exhibited single phase at the addition level of 0.5–2 wt.% for each additives. However, one second phase was detected in the specimens with 3 wt.% Bi2 O3 addition. The phase was not caused by ZnO addition since that the Zn ion did not concentrate in grains but formed the boundary phases such as Zn2 TiO4 [2]. The second phase detected by EDS spectra was Bi rich as compared with the matrix grain. Comparing to XRD, secondary phase was identified as Bi2 Ti2 O7 . The solid solution of Bi in ZST ceramics could be obtained with less than 3 wt.% Bi addition concluded by

The ε r values revealed the same trend as that of the densities of ZST ceramics. In general, higher density resulted in higher dielectric constant owing to lower porosity (the dielectric constant of pore equals 1.0). As CuO amount increased (>1 wt.%), the εr value of ZST ceramics decreased. It could be explained that lower densities caused lower ε r value of ZST ceramics when more CuO was added. Many microwave dielectric loss were suggested including intrinsic loss and extrinsic loss [14]. The intrinsic losses were mainly caused by the lattice vibration modes while the extrinsic losses were dominated by second phases, oxygen vacancies, grain sizes and densification/porosity. Interfacial polarization was thought to play an important role in porous materials. The Q × f value, however, was independent of the density or the porosity for TDs >90%. It could be concluded that the Q × f values were independent of the densification since the densities of the samples investigated in this studies were higher than 95% TDs. Grain sizes were suggested to affect the Q × f values of dielectric resonators [14]. Larger grain resulted in less grain boundary which meant less lattice mismatch and lower dielectric loss. However, the Q × f values demonstrated large variation while the grain sizes remained similar. Related to Figs. 5 and 7, the effect of grain size was also not independent. Bi3+ ions which acted as acceptor and was presented as impurities in the starting materials. The existence of oxygen vacancies in ZST ceramics was due to the oxygen deficiency or the trivalence impurities. Since Bi3+ acts as an acceptor, the reaction could be expressed as [14,15] ••

Bi2 O3 → 2BiTi + 3Oo + Vo

(2)

Table 2 Physical and dielectric properties of ZST ceramics with additions sintered at 1300◦ Ca Additions

Amount (wt.%)

Density (g cm−3 )

TD (%)

GS (␮m)

εr

Q×f

τ f (ppm ◦ C−1 )

CuO

0.5 1 2

4.96 5.08 5.01

95.8 97.7 96.3

5.1 5.4 5.2

35.2 37.5 37.2

28000 45000 26000

1.3 0.5 −1.2

Bi2 O3

0.5 1 2 3

5.06 5.06 5.08 5.03

97.1 97.3 97.7 96.7

4.2 4.5 5.0 5.3

38.6 39.0 39.0 37.5

31160 28000 15000 NA

−1.8 −2.0 −2.2 −3.4

V 2 O5

0.5 1 2

5.01 4.98 4.96

96.3 95.8 95.4

5.0 5.02 5.03

38.2 37.2 36.9

36500 51200 41500

−1.4 −2.1 −3.8

a

TD, relative density; GS, grain sizes; NA, not available.

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The decrease of Q × f values of ZST ceramics with Bi2 O3 was attribute to the increase of oxygen vacancies which increased the anharmonic interaction. The significant decrease of Q×f value in the case of 3 wt.% Bi2 O3 addition could be due to the formation of second phase, as shown in Fig. 2. In addition, the second phase seemed more harmful to the Q × f value than the increase of oxygen vacancies in ZST ceramics. Similarly, the Q × f value of ZST ceramics increased with increasing V2 O5 addition up to 1 wt.% and then deceased. Since V5+ act as a donor, the reaction can be represented by the following formula. ••

•

V2 O5 + Vo → 2VTi + 5Oo

(3)

The increase of Q × f value was due to the decrease of oxygen vacancies which decreased the anharmonic interaction. Further increasing the amount of V2 O5 in ZST ceramics would result in the decrease of Q × f value. 5. Conclusion The effects of V2 O5 , Bi2 O3 and CuO additives on ZST ceramics were investigated. ZST ceramics with proper V2 O5 , Bi2 O3 or CuO additive can be well sintered to approach 96% TDs at 1300◦ C due to liquid phase effect observed. At low addition level (<3 wt.%), the (Zr0.8 Sn0.2 )TiO4 ceramics remained single phase and presented second phase while added with 3 wt.% Bi2 O3 . The second phase would seriously decreased the dielectric properties. The sintering temperatures of ZST ceramics were effectively reduced to 1300◦ C and the grain were growed with CuO, Bi2 O3 or V2 O5 additives. The dielectric constants were mainly related to the densities of ZST ceramics. The temperature coefficient of resonant frequency (τ f ) of ZST ceramics slightly shifted to negative with increasing amount of additives and remained values <±3 ppm ◦ C−1 . A number of loss mechanisms influenced the Q × f values of ZST ceramics. The oxygen vacancies were suggested as an important factor on the dielectric loss for ZST ceramics with various additives.

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