**9. ZrO2-TiO2-SnO2 – Based Mw dielectrics**

128 Dielectric Material

Co4Nb2O9

**Figure 13.** Micrographs of microsections of polycrystalline Co1-xNb2O6-x samples with x = 0.05 (a), x = 0.03 (b), x = 0 (c), x = - 0.03 (d), x = - 0.05 (e, f), sintered at 1500 0C for 1h (a - f), 6 h (e): A = Nb2O5, B =

**Figure 14.** Plots of the product Q × ƒ of A1-xNb2O6 samples (where A = Mg (1), Zn (2), Co(3)) against

concentration. The samples were sintered in air for 8 h at 1400 0C (1 and 3) and 1300 0C (2).

The first information about ZrTiO4 as a promising high-Q dielectric was presented in Ref [58]. Later, in the 1950s, investigations of solid solutions in the ZrO2-TiO2-SnO2 system were carried out [59]. It was shown that the composition Zr0.8Sn0.2TiO4 has the highest Q values [60, 61].

ZrTiO4 crystallizes in orthorhombic structure (space group Pbcn) [62, 63, 64] with the space lattice parameters: a = 4.806Å, b = 5.447Å, c = 5.032Å. The unit cell contains two formula units, theoretical density 5.15 g/cm3. It should be noted that in ZrTiO4 there is an order-disorder phase transition in the temperature range 1100-1200°C [65, , , , 69]. When the temperature is decreased, this transition is from an -PbO2 – type high-temperature phase of which disordered arrangement of Zr and Ti ions is typical, to a low-temperature phase with ordered arrangement of Zr and Ti ions [70, 71]. Addition of Sn to ZrTiO4 results in the stabilization of the disordered distribution of cations. The variation of the lattice parameters in the Zr1 xSnxTiO4 system with increasing x is shown in Fig15. As is seen from Fig 15, there are no noticeable changes in the behavior of the parameters a and b in the phase transition region (1100-1200°C) with increasing Sn content. At the same time, as Sn ions are added, a noticeable change in the dependence of the parameter c in the phase transition region is observed [69].

**Figure 15.** Variation of the lattice parameters of Zr1-xSnxTiO4 materials as a function of temperature: ( ) - ZrTiO4; ( ) Zr0,95Sn0,05TiO4; ( ) Zr0,9Sn0,1TiO4; ( ) Zr0,8Sn0,2TiO4; ( ) Zr0,7Sn0,3TiO4 [69].

The phase transition is greatly affect by the cooling rate of Zr1-xSnxTiO4 samples (Fig. 16.). At high cooling rates, the high-temperature disordered phase is frozen in the sample. As the cooling rate is decreased, a noticeable change in the behavior of the parameter *c* is observed, which is connected with increase in the degree of cation ordering [69].

Microwave Dielectrics Based on Complex Oxide Systems 131

ions takes place at domain boundaries, which decrease their contribution to dielectric loss in

Zr0.8Sn0.2TiO4 – based dielectric materials are widely used in the manufacture of various MW equipment elements [63, 83, , 85]. How ever, the manufacture of high-density Zr0.8Sn0.2TiO4 ceramic is a big technological problem even at high temperatures (above 16000C). Therefore, many authors studied the effect of small additions on the sintering temperature and density of Zr0.8Sn0.2TiO4 [85,88]. The dopants ZnO, CuO, Y2O3 are generally used. Addition of small amounts, e.g. of ZnO, results in the formation of a liquid phase at grain boundaries, which increases greatly the ceramic density thanks to fast mass transport through the liquid phase

**10. Ba(M2+1/3 M5+2/3)O3 – Based MW dielectrics (M2+ = Mg, Zn, Co, Ni; M5+ =** 

Ba(B2+1/3 B5+2/3) O3 compounds where B2+ = Mg, Zn, Co, Ni; B5+ = Ta, Nb (perovskite crystal structure) had been synthesized for the first time by the authors of [89,, , 92]. In these compounds, a 2:1- type ion ordering in the B sublattice is observed, in which two layers filled with B5+ ions alternate with a layer filled with B2+ ions. The authors [93, 94] showed that tantalum-containing materials possess a high Q value in the MW range. It should be noted that the synthesis of these materials involves many problems. Ceramics sinter at a high temperature, which may result in considerable evaporation of constituents (cobalt, zinc) and hence in the impairment of electrical properties. In the case of synthesis by the solid-state reaction method, extra phases Ba5Ta4O15, Ba4Ta2O9 are often present in ceramics [95], which affect adversely the Q value. To prevent this, the authors of [96] carried out synthesis from solutions, where solutions containing Mg2+, Ta5+ were used as starting

It had been found that in this case, the single-phase product Ba(Mg1/3Ta2/3)O3 is formed above 1300 0C without intermediate phases. Single-phase Ba(Mg1/3Ta2/3)O3 had been obtained by the solid-state reaction method too, using highly active reagents as starting

When synthesizing tantalum-containing materials, the preparation of high-density ceramics was a difficult problem. Therefore, Nomura with coauthors [98] proposed to prepare dense ceramics (ρ ≈ 7520 kg/m3) by using additionally manganese impurities. Matsumoto and Hinga [99] used fast heating (330 0C/min) for the same purposes, which made it possible to achieve 96% of the theoretical density. To increase the rate of sintering and to order ions in the B sublattice, the authors of [100, 101] proposed preliminarily synthesized MTa2O6(M = Mg, Zn) as starting reagents. Renoult with coworkers [102] had synthesized fine-grained Ba(MgTa)O3 by the sol-gel method. In that case, dense ceramics could be obtained without

The Q value in Ba(B2+1/3 B5+2/3)O3 perovskites is greatly affected by the type and degree of ion ordering in the B sublattice [103]. It had been found that by the partial substitution of Zr4+,

substances, to which a solution of ammonia with oxyquinoline was added.

and considerable decrease in sintering temperature.

**Ta, Nb) with extremely high Q** 

substances [95-97].

additives.

ceramic.

**Figure 16.** Variation of the parameter *c* of Zr1-xSnxTiO4 ceramic as a function of temperature at different cooling rates: (a) 100°C/h, (b) 15°C/h, (c) 5°C/h, (d) 1°C/h [69].

In the system ZrxSnyTizO4, where x+y+z = 2, single-phase materials are formed in a limited region [62]. The partial substitution of Sn ions for Zr ions stabilized the high-temperature phase with disordered distribution of cations [72] and extends the temperature range of phase transition in ZrTiO4 [73, 74].

Many authors studied the dielectric properties of ZrTiO4 in the MW range and showed it to the have the following parameters: ε = 42, Q×*f* = 28 000 GHz, τf = 58 ppm/°C [62, 63, 75, 76]. The partial substitution of Sn ions for Zr ions affect greatly the dielectric properties of Zr1 xSnxTiO4 materials.

The substitution of Sn ions for Zr ions results in the formation of Zr0.8Sn0.2TiO4, which has good dielectric properties in the MW region: ε = 38, Q×*f* = 49 000 GHz, τf = 0 ppm/°C [77]. This made is possible to use widely this composition in engineering. The use of anatase as a starting reagent instead of rutile made it possible to increase the value of Q×*f* [78]. It should be noted that cation ordering in the ZrTiO4 structure leads to an increase Q×*f*. It was shown that addition of tin leads to a decrease in cation ordering [62, 72, 79, , 81], but in spite of this, the value of Q×*f* increases greatly. Cation ordering is also observed in tin-containing samples, but the ordering domain size decrease with increasing tin content [82]. These data indicate that the increase in Q×*f* on the partial substitution of tin ions for zirconium ions cannot be attributed to cation ordering. The authors of [80] suggested that segregation of tin ions takes place at domain boundaries, which decrease their contribution to dielectric loss in ceramic.

130 Dielectric Material

The phase transition is greatly affect by the cooling rate of Zr1-xSnxTiO4 samples (Fig. 16.). At high cooling rates, the high-temperature disordered phase is frozen in the sample. As the cooling rate is decreased, a noticeable change in the behavior of the parameter *c* is observed,

**Figure 16.** Variation of the parameter *c* of Zr1-xSnxTiO4 ceramic as a function of temperature at different

In the system ZrxSnyTizO4, where x+y+z = 2, single-phase materials are formed in a limited region [62]. The partial substitution of Sn ions for Zr ions stabilized the high-temperature phase with disordered distribution of cations [72] and extends the temperature range of

Many authors studied the dielectric properties of ZrTiO4 in the MW range and showed it to the have the following parameters: ε = 42, Q×*f* = 28 000 GHz, τf = 58 ppm/°C [62, 63, 75, 76]. The partial substitution of Sn ions for Zr ions affect greatly the dielectric properties of Zr1-

The substitution of Sn ions for Zr ions results in the formation of Zr0.8Sn0.2TiO4, which has good dielectric properties in the MW region: ε = 38, Q×*f* = 49 000 GHz, τf = 0 ppm/°C [77]. This made is possible to use widely this composition in engineering. The use of anatase as a starting reagent instead of rutile made it possible to increase the value of Q×*f* [78]. It should be noted that cation ordering in the ZrTiO4 structure leads to an increase Q×*f*. It was shown that addition of tin leads to a decrease in cation ordering [62, 72, 79, , 81], but in spite of this, the value of Q×*f* increases greatly. Cation ordering is also observed in tin-containing samples, but the ordering domain size decrease with increasing tin content [82]. These data indicate that the increase in Q×*f* on the partial substitution of tin ions for zirconium ions cannot be attributed to cation ordering. The authors of [80] suggested that segregation of tin

cooling rates: (a) 100°C/h, (b) 15°C/h, (c) 5°C/h, (d) 1°C/h [69].

phase transition in ZrTiO4 [73, 74].

xSnxTiO4 materials.

which is connected with increase in the degree of cation ordering [69].

Zr0.8Sn0.2TiO4 – based dielectric materials are widely used in the manufacture of various MW equipment elements [63, 83, , 85]. How ever, the manufacture of high-density Zr0.8Sn0.2TiO4 ceramic is a big technological problem even at high temperatures (above 16000C). Therefore, many authors studied the effect of small additions on the sintering temperature and density of Zr0.8Sn0.2TiO4 [85,88]. The dopants ZnO, CuO, Y2O3 are generally used. Addition of small amounts, e.g. of ZnO, results in the formation of a liquid phase at grain boundaries, which increases greatly the ceramic density thanks to fast mass transport through the liquid phase and considerable decrease in sintering temperature.
