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The microstructure and microwave dielectric properties of a (1−x)ZnAl2O4–xTiO2 ceramic system prepared by the mixed oxide route have been investigated. The phases of ZnAl2O4 and TiO2 co-exist with each other and form a two-phase system, which is confirmed by the X-ray diffraction patterns and the energy dispersive X-ray spectrometer analysis. The microwave dielectric properties of the specimens are strongly related to the sintering temperature, the density, and the mole ratio of ZnAl2O4/TiO2. The sintering temperature of the specimen can be effectively lowered by increasing the TiO2 content. The Qu×f values of the ceramics can be significantly boosted by adding an appropriate amount of TiO2 and by sintering at a suitable temperature. Consequently, a very high Qu×f of 277 000 GHz associated with an ɛr of 25.2 and a large resonant frequency (τf) of 177 ppm/°C are obtained using 0.5ZnAl2O4–0.5TiO2 ceramics at 1390°C/4 h. These unique properties can be utilized as a τf compensator for dielectrics that would require extremely low loss. The MgTiO3 and Mg4Nb2O9 having negative τf were mixed with 0.5ZnAl2O4–0.5TiO2 ceramics to achieve dielectrics with a low ɛr, a high Qu×f, and a nearly-zero τf. In addition, a circle dual-mode microstrip bandpass filter is designed and fabricated using the proposed dielectric to study its performance.
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In the last few decades, the microwave-based wireless communications industry has been revolutionized with the use of ceramic dielectric materials to reduce the size and the cost of components in circuit systems due to their unique electrical properties. In particular, size reduction is mainly a result of the use of high dielectric constant materials because the wavelength (λ) in dielectrics is inversely proportional to according to the relation where λo is the wavelength in vacuum. However, as the frequency of interest is extended from industrial, scientific, and medical (ISM) bands to the millimeter wave range, materials with a high dielectric constant (ɛr) tend to become of less interest.1 Consequently, a high quality factor (Q) together with a low ɛr would play a more prominent role instead, because the high Q can significantly reduce the dielectric loss and low ɛr, which allows a fast time for electronic signal transition at ultrahigh frequencies. Zero resonant frequency (τf) is also one of the major requirements for dielectric materials to be utilized as a frequency-stable passive component. The most convenient and promising way to achieve a zero τf is to combine two compounds having negative and positive τf values to form a solid solution or mixed phases.2,3 However, high ɛr materials exhibit a high dielectric loss (low Qu×f value) and a large positive τf,4,5 while low-loss ceramics are usually accompanied by a low ɛr value and a negative τf.4,6,7 Consequently, it becomes a trade-off problem when mixing two compounds having opposite τf values. For instance, mixing MgTiO3 (ɛr=17, Qu×f∼160 000 GHz, τf∼−50 ppm/°C) and CaTiO3 (ɛr=170, Qu×f∼3600 GHz, and τf∼800 ppm/°C) would lead to a compromised combination of dielectric properties (ɛr=21, Qu×f∼56 000 GHz, and τf∼0 ppm/°C) for 0.95MgTiO3–0.05CaTiO3.4
Spinel zinc aluminate (ZnAl2O4) is a well-known material for a catalyst.8 Its microwave dielectric properties (ɛr=8.5, Qu×f∼56 300 GHz, and τf∼−79 ppm/°C) were firstly investigated by Surendran et al.9 By adding appropriate rutile TiO2, they had suggested that a fine combination of dielectric properties (ɛr=12.67, Qu×f∼100 000 GHz, and τf∼0.74 ppm/°C) could be achieved with the 0.83ZnAl2O4–0.17TiO2 ceramic. Lei et al.10 also reported a compatible result (ɛr=11.4, Qu×f∼71 810 GHz, and τf∼−0.5 ppm/°C) in the 0.79ZnAl2O4–0.21TiO2 compound. Both of them were fired at temperatures as high as 1425°C. Further modifications on the ZnAl2O4–TiO2 dielectrics were also performed by both groups to lower its ɛr by replacing TiO2 with M2TiO4 (M=Mg, Co, and Mn) or filler-added PTFE.11–13
Instead of searching for a low ɛr material, we seek to develop a dielectric having a low ɛr,14–16 a high Qu×f, and particularly, a large positive τf, so that it can act simultaneously as a compensator for τf to avoid encountering the aforementioned trade-off problem. Therefore, the dielectric properties of the ZnAl2O4–TiO2 system were closely investigated and discussed in terms of the compositional ratio, the densification, and the sintering temperature of the specimens. X-ray diffraction (XRD) and scanning electron microscopy (SEM) analysis were also used to study the crystal structures and microstructures of the ceramics. In addition, compounds having negative τf values, such as MgTiO3 and Mg4Nb2O9, were mixed with the ZnAl2O4–TiO2 ceramics to achieve dielectrics with a low ɛr, a high Qu×f, and a near-zero τf.
II. Experimental Procedure
The starting materials were high-purity oxide powders (>99.9%): Al2O3, ZnO, and TiO2. The powders were prepared with the desired stoichiometry (1−x)ZnAl2O4–xTiO2 and the mixtures were ground in distilled water for 6 h in a ball mill with agate balls. The powders were dried and calcined at 1100°C for 4 h in air. Prepared powders were re-milled for 12 h, together with 5 wt% of a 10% PVA solution as a binder (polyvinyl alcohol 500, Showa, Tokyo, Japan), and pressed into pellets 11 mm in diameter and 5 mm in thickness. All samples were prepared using an automatic uniaxial hydraulic press at 200 MPa. These pellets were sintered at 1270°–1510°C for 4 h in air.
The crystalline phases of the sintered ceramics were identified by XRD using CuKα (λ=0.15406 nm) radiation with a Siemens D5000 diffractometer (Siemens, Munich, Germany) operated at 40 KV and 40 mA. Microstructural observation and analysis of the sintered surface were performed using SEM (Philips XL_40FEG, Philips, Eindhoven, the Netherlands) and an energy-dispersive X-ray spectrometer. The apparent densities of the sintered pellets were measured by the Archimedes method. The ɛr and the quality factor values (Q) at microwave frequencies were measured using the Hakki–Coleman dielectric resonator method.17,18 A system combining an HP8757D network analyzer and an HP8350B sweep oscillator (HP, Palo Alto, CA) was used in the measurement. For the temperature coefficient of τf, the technique is the same as that of Q measurement. The temperature coefficient of τf at microwave frequencies was measured in the temperature range from 25° to 80°C and is defined by
where Δf0 is the shift in the center frequency introduced by temperature change ΔT. In addition, the accuracy of the measurement techniques used in the experiment was verified to be ±5%.
III. Results and Discussion
Preparation of samples was accomplished by sintering the (1−x)ZnAl2O4–xTiO2 ceramics at temperatures 1270°–1510°C. Figure 1 shows the XRD patterns recorded from 0.5ZnAl2O4–0.5TiO2 sintered at different temperatures. The XRD patterns show a two-phase system with a spinel ZnAl2O4 and a rutile TiO2. Additional phase formation is not detected throughout the complete range of mixtures. In order to provide a clear inspection on the influence of the compositional ratio over the phase formation of the specimens, the XRD patterns of the (1−x)ZnAl2O4–xTiO2 ceramics at 1390°C for 4 h were also recorded and are shown in Fig. 2. At x=0.1, clear ZnAl2O4 is detected as a major phase, while the TiO2 phase is barely observed. However, the intensity of the TiO2 phase starts to increase continuously with an increase in the TiO2 content. It is not until x reaches 0.9 that ZnAl2O4 starts to become a minor phase. It also indicates that a two-phase system and its phase formation can be controlled by the compositional ratio.
Figure 3 shows the SEM micrographs of the specimens using 0.5ZnAl2O4–0.5TiO2 ceramics at different temperatures for 4 h. Energy-dispersive X-ray (EDX) analysis was also used in combination with SEM to distinguish each grain for 0.5ZnAl2O4–0.5TiO2 ceramics, as shown in Fig. 3(b). The grain morphology of the specimens can be grouped into two types: larger grains (Spot A) are identified as TiO2 and smaller ones (Spot B) are represented as the ZnAl2O4 phase (Table I). As illustrated in Fig. 3, the result indicates that the specimen does not appear dense and the grain is not grown at 1330°C. In addition, the grain size increases as the sintering temperature increases. The pores are almost eliminated for the specimen sintered at 1360°C, and a noticeable grain growth and a relatively uniform surface morphology are observed at 1390°C. However, inhomogenous grain growth is monitored at temperatures higher than 1390°C, which might degrade the microwave dielectric properties of the ceramics. The melting points of TiO2 and ZnAl2O4 are 1830° and 1980°C, respectively. The sintering temperature of a ceramic is normally 75% of its melting point.19 According to this rule, the sintering temperatures of TiO2 and ZnAl2O4 should be around 1373° and 1485°C, respectively. Either way, a lower firing temperature for TiO2 is demonstrated. It implies that overheating the specimen may lead to an over-grown TiO2 grain, which would result in an abnormal grain growth. Moreover, by taking the apparent size difference in grains into consideration, a mixture of two phases along with an appropriate ratio could effectively hold back the abnormal grain growth as a result of rapid grain growth.20,21 This explained why the average grain size of TiO2 in our experiment was smaller than that reported by Lei et al.10 under similar conditions. The micrographs of the specimens using (1−x)ZnAl2O4–xTiO2 ceramics at 1390°C/4 h are illustrated in Fig. 4. The introduction of ZnAl2O4 does inhibit the grain growth of TiO2. With the ratio of ZnAl2O4:TiO2=1:1, a relative uniform grain growth could be achieved for the specimen sintered at 1390°C.
Table I. The Energy Dispersive X-ray Analysis (EDX) Data of the (0.5ZnAl2O4–0.5TiO2) Ceramics Corresponding to Fig. 3(b)
Figure 5 demonstrates the relative density of the (1−x)ZnAl2O4–xTiO2 ceramics as a function of its sintering temperature for 4 h. Initially, the densities apparently increase with increasing sintering temperature. After reaching their maximum, they start to decrease. However, the breaking points appear at different temperatures for specimens with various x values. The sintering temperature of the specimen seems to be effectively lowered by increasing the TiO2 content. The decrease in the density is due to the inhomogeneous grain growth as shown in Fig. 3. In addition, the theoretical densities of ZnAl2O4 and TiO2 are 4.58 and 4.26 g/cm3, respectively. The maximum relative densities of specimens at different x values are all >98%. In addition, the highest relative density (98.5%) appears at x=0.5, implying an appropriate ratio of ZnAl2O4/TiO2 chosen for a highly dense specimen. Similar results were reported by Surendran et al.9 and Lei et al.10 for specimens with x=0.1 and 0.3 at ∼1480°C.
Figure 6 shows the ɛr of the (1−x)ZnAl2O4–xTiO2 ceramics as a function of its sintering temperature administered for a duration of 4 h. The variation of the ɛr is consistent with that of the density. For instance, the ɛr of the specimen with x=0.1 increased from 9.01 at 1390°C to 10.54 at 1480°C and decreased thereafter. In addition, the ɛr increases in response to an increase in the TiO2 content because TiO2 possesses a higher dielectric constant (ɛr=100).19 The ɛr values of the specimens were found to be compatible with earlier reports by Surendran et al.9 and Lei et al.10
The Qu×f value of the (1−x)ZnAl2O4–xTiO2 ceramics as a function of its sintering temperature conducted for 4 h is illustrated in Fig. 7. On increasing the sintering temperature, the Qu×f value increases to a maximum value and decreases thereafter. According to Templeton et al.,22 the use of divalent and trivalent dopant ions only, with an ionic radius in the range of 0.5–0.95 Å, resulted in a TiO2 ceramic with an increased Q value. As indicated previously, both Zn (radius of Zn2+=0.74 Å) and Al (radius of Al3+=0.535 Å) were detected in the TiO2 grains, implying the formation of high-Q-doped TiO2. Its Qu×f value could become as high as 50 000 GHz. Consequently, ZnAl2O4 and high-Q TiO2 have comparable Qu×f values. Still, all of the compositions in the (1−x)ZnAl2O4–xTiO2 system have higher Q than those of the end members. Similar results were reported by Surendran et al.9 and Lei et al.10 In comparison with their results, compatible Qu×f values were obtained for specimens with x=0.1 and 0.3 at 1480°–1510°C. For x=0.5 and 0.7, however, a relatively high Qu×f was attained at 1390°C because of a higher density as well as a more uniform surface morphology, although it might also fall back to a lower Qu×f on increasing the sintering temperature to 1480°–1510°C through interpolation. The maximum Qu×f value of 277 000 GHz was obtained for 0.5ZnAl2O4–0.5TiO2 ceramics sintered at 1390°C for 4 h. Densification of the ceramics plays an important role in controlling the dielectric loss and the same phenomenon has also been shown for other microwave dielectric materials. The decrease in Qu×f at high temperatures is mainly due to the low density. In conclusion, the Qu×f value of the (1−x)ZnAl2O4–xTiO2 ceramics is mainly dominated by the density and the surface morphology.
Figure 8 demonstrates the temperature coefficients of τf of the (1−x)ZnAl2O4–xTiO2 ceramics as a function of its sintering temperature for 4 h. The τf is almost independent of the sintering temperature because no significant compositional change is observed. On increasing TiO2, the τf value varies linearly in a positive direction owing to a large positive τf value (450 ppm/°C)22 rendered by TiO2. The τf of 177 ppm/°C can be obtained for the 0.5ZnAl2O4–0.5TiO2 specimen at 1390°C for 4 h.
To verify the performance of the proposed material as a compensator for τf, MgTiO3, and Mg4Nb2O9 were individually mixed with the 0.5ZnAl2O4–0.5TiO2 ceramics to achieve dielectrics with a low ɛr, a high Qu×f, and a nearly-zero τf. Table II illustrates the microwave dielectric properties of the ceramic mixtures. Consequently, τf of the specimen can be effectively compensated while still retaining an extremely high Qu×f value. An additional phase was not detected for these compositions. In addition, a circle dual-mode microstrip bandpass filter was designed and fabricated on different dielectric substrates, namely, FR4, alumina, and (MgTiO3)–(0.5ZnAl2O4–0.5TiO2). Figure 9 shows the physical layout and the fabricated filters designed with a central frequency of 2.5 GHz, and the measurement results are illustrated in Table III. In comparison with FR4 and alumina, the filter using the proposed dielectric not only shows a tremendous reduction in the insertion loss but also demonstrates a considerable reduction in its size.
Table II. Microwave Dielectric Properties of (0.5ZnAl2O4–0.5TiO2)-Based Ceramic System
0.47Mg4Nb2O9–0.53(0.5ZnAl2O4–0.5TiO2) at 1390°C/4 h
Table III. Measurement Results of the Bandpass Filters Using Different Dielectrics
Central frequency (GHz)
Insertion loss (dB)
Return loss (dB)
The sintering behavior and dielectric properties of ZnAl2O4–TiO2 ceramics have been investigated in this paper. ZnAl2O4 with a smaller grain size could effectively hold back the grain growth of TiO2. It would lead to a higher density, resulting in a lower dielectric loss of the specimen. Moreover, the microwave dielectric properties are strongly dominated by the density, the ZnAl2O4/TiO2 ratio, and the surface morphology. A dielectric constant (ɛr) of 25.2, a high Qu×f value of 277 000 GHz, and a large τf of 177 ppm/°C can be obtained for 0.5ZnAl2O4–0.5TiO2 ceramics sintered at 1390°C for 4 h. It can be used as an appropriate τf compensator for dielectrics that require a low ɛr, a high Qu×f, and a nearly-zero τf. By individually adding MgTiO3 and Mg4Nb2O9 to the proposed τf compensator, dielectrics with a Qu×f of as high as 210 000 GHz could be achieved. In comparison with FR4 and alumina, the filter using the compensated dielectric not only shows a tremendous reduction in the insertion loss but also demonstrates a significant reduction in its size.