1 Introduction

In recent years, with the rapid development of advanced communication systems, the microwave dielectric materials have been widely used for microwave device applications, such as resonators, filters, and oscillators [1–4]. To meet the demands of microwave circuit designs, microwave dielectric resonator ceramics with a high dielectric constant (εr > 20) for miniaturization, a high-quality factor (Q > 2,000) for better selectivity, and a near zero temperature coefficient of the resonant frequency (τ f ) for stable frequency stability are required [5–7]. Research of new dielectric materials that satisfy such a requirement has been ongoing, while the study for promoting the dielectric properties of currently available materials through appropriate modifications has been also needed.

In the ZnO–TiO2 system, there are three compounds existing: Zn2TiO4 with a face-centered cubic crystal structure, Zn2Ti3O8 with a simple cubic structure, and ZnTiO3 with a rhombohedral structure [8–13]. ZnTiO3 is an attractive material for applications owing to its low loss at high frequency. However, ZnTiO3 is unstable at high temperature and decomposes into Zn2TiO4 and TiO2 at temperatures above 945 °C [10, 14–16]. Therefore, many efforts have been made to prepare ZnTiO3 with improved stability and microwave dielectric properties. The recent studies for A-sites substitutions on the microwave dielectric properties of Zn1−xAxTiO3 (A = Ni, Mg, Co, Mn) ceramics were studied by researchers [10, 11, 17]. For instance, the microwave dielectric properties of the Zn0.3Co0.7TiO3 specimen with εr ≈ 23, Q × f ≈ 80,000 GHz and τ f  ≈ −50 ppm/°C was reported by Kim and Lanagan [10]. Although the high-quality factor of the ceramics were obtained, the large τ f values limited the use for practical applications. A near zero τ f remains as one of the primary requirements for microwave materials and becomes more and more critical as the operating frequency going higher. Therefore, the further studies of the Zn1−xAxTiO3 (A = Ni, Mg, Co, Mn) ceramics to achieve a zero τ f are necessary. Since the ionic radius of Ti4+ (0.605 Å, CN = 6) is similar to that of Sn4+ (0.69 Å, CN = 6) [18–20], the Ti4+ ion can be partially substituted by Sn4+ ion to form a new solid solutions, which may enhance the thermal stability and dielectric properties [19, 20]. In this work, the effects of Sn4+ substitution on the microwave dielectric properties of the (Zn0.3Co0.7)Ti1−xSnxO3 (x = 0–0.14) ceramics due to the changes in chemical composition, density, and microstructure have been investigated.

2 Experimental

The (Zn0.3Co0.7)Ti1−xSnxO3 (x = 0, 0.02, 0.05, 0.09, 0.14) ceramics were synthesized by the solid-state ceramic route. High-purity powders of ZnO (99.7 %), Co2O3 (99.8 %), SnO2 (99.9 %) and TiO2 (99.9 %) were used as the starting materials. The raw oxide materials were weighted according to the stoichiometry proportions and ball milled in an alcohol medium for 6 h in a plastic bottle using zirconia balls. The mixture was dried and calcined at 1,000 °C for 4 h, then re-milled again for 3 h. After drying, the dried powder was mixed with a 6 wt% PVA as a binder, then again dried and ground well. The resultant fine powder was axially pressed into cylindrical disks with different thickness of 4 and 11 mm in a diameter under a pressure of 25 MPa. These pellets were sintered at different sintering temperatures from 1,190 to 1,320 °C for 4 h in air.

After sintering, the bulk densities of the samples were measured using the Archimedes method. The phase composition and crystal structure of the ceramics were examined by X-ray diffraction technique using CuKa radiation (DX-1000 CSC, Japan). Scanning electron microscopy (SEM) (JSM-6460LV, Jeol, Tokyo, Japan) coupled with energy dispersive X-ray spectroscopy (EDS) was employed to study the surface morphology of the specimens. The microwave dielectric properties were measured by a Vector Network Analyzer (E5071C, Agilent Technologies) and a temperature chamber (DELTA 9023, Delta Design, USA). In addition, the temperature coefficients of resonant frequency τ f values were calculated by the equation: \(\tau_{f} = (f_{{t_{2} }} - f_{{t_{1} }} )/(f_{{t_{1} }} \times (t_{2} - t_{1} )),\) where \(f_{{t_{1} }}\) and \(f_{{t_{2} }}\) were the resonant frequencies at the measuring temperature t1 (25 °C) and t2 (85 °C), respectively.

3 Results and discussion

The XRD patterns of (Zn0.3Co0.7)Ti1−xSnxO3 (x = 0, 0.02, 0.05, 0.09, 0.14) ceramics sintered at 1,220 °C for 4 h are shown in Fig. 1. For x = 0, all the diffraction peaks were identified as (Zn,Co)TiO3 phase and (Zn,Co)2TiO4 phase. Cubic spinel phase with (Zn,Co)2(Ti,Sn)O4 and rutile phase TiO2 were observed at x = 0.02–0.05, besides the hexagonal form of ilmenite (Zn,Co)(Ti,Sn)O3. An ilmenite phase disappeared at x = 0.09, only cubic spinel phase (Zn,Co)2(Ti,Sn)O4 and rutile phase TiO2 were obtained at x = 0.09–0.14. This result indicated that the transformation of hexagonal (Zn,Co)(Ti,Sn)O3 phase to cubic (Zn,Co)2(Ti,Sn)O4 and TiO2 was possible at high temperature and the TiO2 phase increased with the increase of Sn content.

Fig. 1
figure 1

The XRD patterns of (Zn0.3Co0.7)Ti1−xSnxO3 (x = 0, 0.02, 0.05, 0.09, 0.14) ceramics sintered at 1,220 °C for 4 h

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Figure 2 depicts the SEM micrographs of the (Zn0.3Co0.7)Ti1−xSnxO3 (x = 0, 0.02, 0.05, 0.09, 0.14) ceramics sintered at 1,220 °C for 4 h. Almost no porosity in compact microstructure was observed in the ceramics. When the content of Sn exceeded 0.05, a significant change was observed in the microstructure, as shown in Fig. 2b–e. This was because the hexagonal (Zn,Co)(Ti,Sn)O3 phase was completely transformed to the cubic (Zn,Co)2(Ti,Sn)O4. To examine the phase compositions, EDS was employed, as shown in Fig. 2a, b, e and Table 1. According to the ratio of the elements, we can deduce that point A and point C represented ZnTiO3-type phase, points B, D and E represented Zn2TiO4-type phase in the ceramic, which were in accord with the XRD result.

Fig. 2
figure 2

The SEM micrographs of (Zn0.3Co0.7)Ti1−xSnxO3 (a x = 0, b x = 0.02, c x = 0.05, d x = 0.09, e x = 0.14) ceramics sintered at 1,220 °C for 4 h

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Table 1 EDS data of the (Zn0.3Co0.7)Ti1−xSnxO3 (x = 0, 0.02, 0.14) ceramic corresponding to Fig. 2a, b, e
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The τ f values of the (Zn0.3Co0.7)Ti1−xSnxO3 (x = 0, 0.02, 0.05, 0.09, 0.14) ceramics sintered at 1,220 °C for 4 h are shown in Fig. 3. The τ f values of the ceramics changed considerably with Sn addition, which increased remarkably from −46.3 to +95.58 ppm/°C. As we know, the temperature coefficient of resonant frequency was related to the composition, additives and second phase of the ceramics [21, 22]. From the XRD result, it was obvious that the TiO2 phase increased with the increase of Sn content (x < 0.09). Therefore, the increase in τ f value in the composition was attributed to the increased amount of TiO2 phase which possessed a τ f value of about +400 ppm/°C [7], compared with the negative τ f phases (spinel and ilmenite: −60 ppm/°C [10]). However, the τ f value stopped increasing at x = 0.09, because the ilmenite (Zn,Co)(Ti,Sn)O3 phase was completely transformed to the cubic (Zn,Co)2(Ti,Sn)O4 and rutile phase TiO2, as shown in Fig. 1 (x = 0.09, x = 0.14). The content of (Zn,Co)2(Ti,Sn)O4 and TiO2 phases stopped increasing at x = 0.09 and 0.14. Therefore, the τ f value kept almost constant at last. Typically, The τ f value of −5.4 ppm/°C was obtained when x = 0.02.

Fig. 3
figure 3

The temperature coefficients of resonant frequency of (Zn0.3Co0.7)Ti1−xSnxO3 (x = 0, 0.02, 0.05, 0.09, 0.14) ceramics sintered at 1,220 °C for 4 h

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The (a) bulk density, (b) dielectric constant (εr) and (c) quality factor (Q × f) of the (Zn0.3Co0.7)Ti1−xSnxO3 (x = 0, 0.02, 0.05, 0.09, 0.14) ceramics as a function of the sintering temperature are illustrated in Fig. 4. It was observed that the bulk densities increased at the beginning of the sintering because of the elimination of pores, then followed by a decrease, which was attributed to the abnormal grain growth. The bulk densities achieved a relative high densities at 1,220 °C when x value varied from 0 to 0.05. However, as the amount of Sn increased above 0.05, the relative high densities of the ceramics were obtained at 1,280 °C, which was related to the composition of the ceramics, as shown in Fig. 1. The variation of the dielectric constant as a function of the sintering temperature showed a similar tendency as the bulk densities, as shown in Fig. 4b. As the amount of Sn increased, the dielectric constant of the samples initially increased, then reached a saturated value at the point x = 0.09, and finally decreased with further increasing the Sn content. From the XRD result, it was obvious that as the content of Sn changed from 0 to 0.09, the rutile phase TiO2 was increased with increasing the Sn content, then the TiO2 content kept almost constant because the hexagonal (Zn,Co)(Ti,Sn)O3 phase completely decomposed into cubic (Zn,Co)2(Ti,Sn)O4 and rutile TiO2. As we know, the dielectric constant of TiO2 was high (εr ≈ 100) [23], compared with the spinel and ilmenite phases (εr ≈ 20) [10]. Therefore, the increase in εr value was attributed to the increase of TiO2 phase. For the sample x = 0.14, the decrease in εr value might be due to the replacing Ti4+ with Sn4+ in (Zn,Co)2(Ti,Sn)O4. From Fig. 4c, it was found that the Q × f value first increased with increasing the temperature and reached the maximum, then declined with the sintering temperature further increased. As the content of Sn increased from 0 to 0.14, the Q × f value initially increased from 56,536 to 66,700 GHz, then decreased. It is well known that the microwave dielectric loss is mainly caused not only by the intrinsic factors but also by extrinsic factors such as the pores, the grain morphology, and the second phases [23–26]. And the ilmenite ZnTiO3 structure has excellent microwave dielectric properties among these phases [15, 27, 28]. Therefore, the increase in Q × f value was due to the formation of the (Zn,Co)(Ti,Sn)O3 solid solution when Ti4+ was partially substituted by Sn4+ in the (Zn,Co)TiO3 ceramic. The quality factor in (Zn0.3Co0.7)Ti1−xSnxO3 (x = 0.05, 0.09, 0.14) composition region was determined by the relative amount of phases between the spinel (Zn,Co)2(Ti,Sn)O4 and ilmenite (Zn,Co)(Ti,Sn)O3 structure [10]. The decrease in Q × f value was attributed to the formation of Zn2TiO4-type structure, which had lower Q × f value than ZnTiO3-type structure [27, 28]. In general, the(Zn0.3Co0.7)Ti1−xSnxO3 (x = 0.02) ceramic sintered at 1,220 °C had good microwave dielectric properties of εr = 24, Q × f = 66,700 GHz and τ f  = −5.43 ppm/°C.

Fig. 4
figure 4

a Bulk density, b dielectric constant and c quality factor of (Zn0.3Co0.7)Ti1−xSnxO3 (x = 0, 0.02, 0.05, 0.09, 0.14) ceramics as a function of the sintering temperature

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4 Conclusions

The effects of Sn ratio on phase composition, microstructure, and the microwave dielectric properties of (Zn0.3Co0.7)Ti1−xSnxO3 (x = 0, 0.02, 0.05, 0.09, 0.14) ceramics were investigated. The results revealed that three crystalline phases were identified in the ZCTS (x = 0, 0.02, 0.05, 0.09, 0.14) ceramics: (Zn,Co)TiO3 + (Zn,Co)2TiO4 phases at x = 0, (Zn,Co)(Ti,Sn)O3 + (Zn,Co)2(Ti,Sn)O4 + TiO2 phases at x = 0.02 and 0.05, (Zn,Co)2(Ti,Sn)O4 and TiO2 phases at x = 0.09 and 0.14. As the amount of Sn increased, the Q × f value of the samples initially increased because of the formation of the (Zn,Co)(Ti,Sn)O3, then reached a saturated value, and finally decreased with further increasing the Sn content. Due to the compensating effect of rutile TiO2 (τ f  ≈ +400 ppm/°C), the temperature coefficient of resonant frequency (τ f ) for (Zn0.3Co0.7)Ti1−xSnxO3 (x = 0.02) ceramic was tuned to near zero value. Typically, the (Zn0.3Co0.7)Ti1−xSnxO3 (x = 0.02) ceramic sintered at 1,220 °C for 4 h exhibited good microwave dielectric properties of εr = 24, Q × f = 66,700 GHz and τ f  = −5.43 ppm/°C.