**14. Conclusion**

140 Dielectric Material

**Figure 22.** Coaxial dielectric resonators

**Figure 23.** Open dielectric resonators

should tend to zero; it is defined as:

vibration modes such as whispering gallery modes.

frequency of dielectric resonator in the temperature range ΔT.

over 1000. Since open resonators have a high Q value, they can be used in the decimeter

The size of open dielectric resonators operating on the characteristic modes TE01s and H01s becomes very small at frequencies above 30 GHz, which makes their use in this band impossible. Therefore, at frequencies above 30 GHz, it is expedient to use extraordinary

The modern communication systems in which dielectric resonators are used operate in a temperature range of – 40-80 0C. Therefore, high thermostability of dielectric resonator resonance frequency is required. It is necessary that the temperature coefficient of frequency

> 1 *<sup>p</sup> p <sup>f</sup> TK*

(9)

*f T*

where ƒr is the resonance frequency of dielectric resonator, Δ ƒr is change in the resonance

wave band (about 1 GHz), though in this case their size becomes large.

High-Q MW dielectrics with high thermostability of electrophysical properties can be developed on the basis of single-phase and multiphase systems. Single-phase MW dielectrics are produced on the basis of solid solutions [6, 15, 98] using heterovalent substitutions in one of the crystal sublattices and influencing thereby the phonon spectrum [10] and by making one of the sublattices "mobile" [151]. At the same time, high-Q thermostable MW dielectrics based on multiphase systems are developed using the volume temperature compensation effect [55-57, 124, 132, 135].

During the last decade, MW dielectrics with increased permittivity (ε ≥ 10) contribute, to a larger measure than other factors, to considerable miniaturization and reduction in the price of modern communication systems. It should be noted that there is still a great potential for further microminiaturization and reduction of prices of modern communication systems thanks to the use of components made on the basis of MW dielectrics.

Microwave Dielectrics Based on Complex Oxide Systems 143

Thus, the synthesis of novel high-Q MW dielectrics and the investigation of their structure

*V.I. Vernadskii Institute of General and Inorganic Chemistry of the Ukrainian NAS, Kyiv, Ukraine* 

[1] Poplavko Yu.M., (1980) Physics of Dielectrics (in Russian), Vyshch. Shk., Kyiv 325. [2] Poplavko Yu.M., Belous A.G., (1984) Physical background of the temperature stability

[3] Chool-Woo Ahu, Hyun-Jung Jang, Sahn Nahm et al. (2003) Effects of microstructure on the microwave dielectric properties of Ba(Co1/3Nb2/3)O3 and (1−x)Ba(Co1/3Nb2/3)O3–

[4] J-Nan Lin, Chih-Ta Chia, Hsiang-Lin Liu et al. (2002) Dielectric Properties of xBa(Mg1/3Ta2/3)O3-(1-x)Ba(Mg1/3Nb2/3)O3 Complex Perovskite Ceramics. Jpn. J. Appl.

[5] Cheol-Woo Ahn, Sahn Hahm, Seok-Jin Yoou et al. (2003) Microstructure and Microwave Dielectric Properties of (1−*x*)Ba(Co13Nb23)O3–*x*Ba(Zn13Nb23)O3 Ceramics

[6] Nenesheva E. A., Mudroliuba L. P., and Kartenko N. F. (2003) Microwave dielectric properties of ceramics based on CaTiO3–LnMO3 system (Ln=La, Nd; M=Al, Ga). J. Eur.

[7] Moon J. H., Jung H. M., Park H. S., Shin J. Y., and Kim H. S. (1999) Sintering behaviour and microwave dielectric properties of (Ca,La)(Ti,Al)O3 ceramics. Jpn. J. Appl. Phys. 38:

[8] Belous A.G., Butko V.I., Novitskaya G.N. et al. (1985) Dielectric spectra of the

[9] Belous A.G., Butko V.I., Polyanetskaya S.V. (1984) Electrical parameters of the solid

[10] Belous A.G (1998) Physicochemical aspect of the development of new functional materials based on heterosubstituted titanates of rare-earth elements with the

[11] Butko V.I., Belous A.G., Yevtushenko N.P. (1986) Vibration spectra of the perovskites

[12] Knyazev A.S., Poplasvko Ye.M., Zakharov V.P., Alekseev V.V. (1973) Soft mode in the

[13] Ohsato H., Nishigaki S., Okuda T. Superlattice and Dielectric Properties of BaO-R2O3- TiO2 (R=La, Nd and Sm) Microwave Dielectric Compounds //Jpn. J. Appl. Phys. -1992. -

of microwave dielectrics. Dielektriki I Poluprovodniki. 25: 3-15.

xBa(Zn1/3Nb2/3)O3 ceramics J. Eur. Ceram. Soc. 23: 2473-2474.

perovskites La2/3-XM3XTiO3 Fizika Tv. Tela. 27: 2013-2016.

solutions of rare-earth titanates Ukr. Khim. Zhurn. 50: 1139-1142.

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vibration spectrum of CaTiO3 Fiz. Tverd.Tela. 15: 3006-3010.

La2/3-xM3xTiO3 Fizika Tv. Tela. 28: 1181-1183.

and properties are an important scientific-technical trend in solid-state chemistry.

**Author details** 

**15. References** 

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A.G. Belous

Depending on the frequency range of modern communication systems, MW dielectrics with different properties are needed. In the decimeter wave band, high permittivity values (ε ≥ 100) are required along with the high thermostability of electrophysical properties and high Q, which enables effective solution of microminiaturization problems. At the present time, solid solutions based on barium-lanthanide titanates (Ba6-xLn8+2x/3Ti18O54 (Ln = La-Gd)), which have a potassium-tungsten bronze structure and ε ≈ 80-100, meet best these requirements. However, the nature of the thermostability of the electrophysical properties of these solid solutions has not been elucidated definitively; there are only qualitative explanations, which greatly restrains the search for new promising MW dielectrics with high permittivity (ε ≥ 100). The presence of spontaneous polarization in dielectrics causes, along with increase in ε, a considerable increase in dielectric loss (the Q value decreases), which impairs greatly the technical characteristics of communication system elements based on them. Therefore, the acquirement of fundamental knowledge, which is required for obtaining thermostable high-Q materials with ε ≥ 150-200, is the most important problem in developing modern decimeter wave band communication systems.

In the centimeter and millimeter wave bands, where the electromagnetic wavelength is much smaller as compared with the decimeter wave band, thermostable MW dielectrics with extremely high Q values are required. In this case, permittivity values may be relatively low (ε = 15-30). To date, tantalum-containing perovskites possess the highest Q values. However, the difficulty of their preparation and the high price call for search for new promising compounds, and it is going on in several directions. In particular, research is now under way to develop niobium-containing perovskites and to create multiphase systems, in which volume temperature compensation effect is realized. It is these directions that will probably be major directions in the next few years in developing high-Q centimeter and millimeter wave band MW dielectrics, though the search for new promising compounds will always be vital. Quite a number of problems pertaining to solid-state physics and chemistry will have to be solved. For instance, it is necessary to investigate the nature of extrinsic loss, which is coupled with various structural defects, as well as with the presence of grain boundaries, ordering of crystal sublattices and domain nanostructure. Research aimed at developing thermostable dielectrics, which will be used as millimeter wave band dielectric resonators using whispering gallery modes, will be of special scientific and practical interest. This requires considerable increase of the chemical and structural homogeneity of dielectrics.

An important problem is the creation of retunable resonant elements. To this end, multilayer bulk and film materials, which will contain a thermostable dielectric phase and a nonlinear magnetic or electrical phase at the same time, will probably have to be developed.

Thus, the synthesis of novel high-Q MW dielectrics and the investigation of their structure and properties are an important scientific-technical trend in solid-state chemistry.
