Volume 21 Issue 5 - February 17, 2012 PDF
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Low Dielectric Loss Ceramics in the ZnAl2O4–TiO2 System as a τf Compensator
Cheng-Liang Huang*, Tung-Jung Yang, and Chung-Chia Huang
Department of Electrical Engineering, College of Electrical Engineering and Computer Science, National Cheng Kung University
 
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In the last few decades, the microwave-based wireless communications industry has been revolutionized by using the 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 from the use of high-dielectric-constant material since 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 ISM (industrial, scientific and medical) bands to millimeter wave range, materials with high dielectric constant tend to become a less of interest. Consequently, high quality factor together with low dielectric constant would play a more prominent role instead, since high quality factor can significantly reduce the dielectric loss and low dielectric constant which allows a fast time for electronic signal transition at ultra high frequencies. Zero 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. However, high dielectric constant materials exhibit high dielectric loss (low Qu×f value) and large positive τf, while low loss ceramics are usually accompanied by low εr value and negative τf. Consequently, it becomes a trade off problem when mixing two compounds having opposite τf values. For instance, mixing MgTiO3 (εr = 17, Qu×f ~160,000 GHz, τf ~ -50 ppm/oC) and CaTiO3 (εr = 170, Qu×f ~3,600 GHz, τf ~ 800 ppm/oC) would lead to a compromised combination of dielectric properties (εr = 21, Qu×f ~56,000 GHz, τf ~ 0 ppm/oC) for 0.95MgTiO3-0.05CaTiO3.

Instead of searching for a low dielectric constant material, we seek to develop a dielectric having low εr, high Qu×f, and particularly, a large positive τf, so that it can be act simultaneously as a compensator for τf to avoid encountering of the aforementioned trade off problem. Therefore, the dielectric properties of ZnAl2O4–TiO2 system were closely investigated and discussed in terms of the compositional ratio, the densification and the sintering temperature of the specimens. The phases of ZnAl2O4 and TiO2 co-exist with each other and form a two-phase system, which is confirmed by the XRD patterns and the EDS analysis. The microwave dielectric properties of the specimens are strongly related to the sintering temperature, the density, and the mole ratio of ZnAl2O4/TiO2. Sintering temperature of specimen can be effectively lowered by increasing TiO2 content. The Qu×f values of the ceramics could be significantly boosted by adding appropriate amount of TiO2 and sintered at a suitable temperature. Consequently, a very high Qu×f of 277,000 GHz associated with a ε of 25.2 and a large τf of 177 ppm/oC are obtained using 0.5ZnAl2O4-0.5TiO2 ceramics at 1390oC/4 h. These unique properties can be utilized as a τf compensator for dielectrics which would require extremely low loss.

Table 1 Microwave dielectric properties of (0.5ZnAl2O4–0.5TiO2)-based ceramic system
No.
Composition
εr
Qu×f(GHz)
τf(ppm/oC)
1
 
 
MgTiO3
17
160,000
-50
18
140,000
-50
2
 
Mg4Nb2O9
12.4
192,000
-70.5
11
210,000
-70
3
(0.5ZnAl2O4–0.5TiO2)
25.2
277,000
177
4
0.23MgTiO3–0.77(0.5ZnAl2O4–0.5TiO2) at 1360oC/4 h
18.7
190,000
-1.8
5
0.47Mg4Nb2O9–0.53(0.5ZnAl2O4–0.5TiO2) at 1390oC/4 h
13.4
210,000
1.8

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 low εr, high Qu×f, and nearly zero τf. Table 1 illustrates the microwave dielectric properties of the ceramic mixtures. Consequently, τf of 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). Fig. 1 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 2. 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.
Fig. 1 The (a) physical layout of the designed circle dual-mode microstrip bandpass filter with a central frequency of 2.5 GHz, and fabricated ones on substrates using (b) FR4, (c) alumina, and (d) presented dielectric.

Table 2 Measurement results of the bandpass filters using different dielectrics
Dielectrics
FR4
Alumina
Presented #4
εr
4.5
9.7
18.7
Tanδ
0.015
0.0001
0.000013
Central freq. (GHz)
2.52
2.48
2.55
Insertion loss (dB)
-3.8
-2.5
-0.8
Return loss (dB)
-27
-31
-37
Bandwidth (MHz)
220
200
205
Size (mm2)
632
400
149
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