1 Introduction

In recent years, along with the rapid growth in wireless communications industry, a variety of microwave devices such as filters, duplexers, resonators and antennas were well developed [1]. Low-temperature co-fired ceramic (LTCC) technology make it possible to integrate many kinds of electronic components in a compact multilayer ceramic structure [2], and silver has been widely used as the internal electrode in the multilayer devices. To realize cofiring with Ag electrode, the processing temperature of the microwave dielectric ceramics must be below 960°C because the melting point of Ag is 960°C [3, 4].

The ZnTiNb2O8 ceramics exhibit an excellent quality factor of 42,500 GHz and dielectric constant of 34 [5], which would be one of the new materials suitable for the applications at microwave frequencies. However, the larger negative temperature coefficient of resonant frequency (τ f  = −52 ppm/°C) and higher sintering temperature (1250°C) restrict its application in co-firing ceramic device. Materials with large positive τ f such as TiO2 (τ f  = + 450 ppm/ °C) were usually used to tune the temperature coefficient to near zero [6]. D.W. Kim et al. reported that ZnO–2TiO2–Nb2O5 ceramics have suitable relative permittivity of 58, Q × f value of 16,300 GHz and τ f of 10 ppm/ °C [7] at 1100 °C. To reduce the sintering temperatures, the low-melting glasses and low-melting oxide composites are always added to ceramics [8, 9].

Reaction-sintering process is a simple and effective route to synthesize ceramics. The calcination step is by passed and the mixture of the materials is sintered directly [10]. In our previous work, ZnO–Nb2O5–2TiO2 (hereafter referred to as ZTN) composite ceramics were prepared by reaction-sintering process at 1100 °C [11]. In this paper, the CuO–V2O5 (hereafter referred to as CV) were employed as sintering aid for its lower eutectic point (630 °C) [12–14]. The effects of CV addition on phase compositions, sinterablity, and microwave dielectric properties of ZTN composite ceramics were investigated.

2 Experimental procedures

CV co-doped ZTN ceramics were prepared by reaction-sintreing method. High-purity oxide powders (≥99 %) were used as raw materials. Stoichiometric starting powders according to the compositions of ZnO–Nb2O5–2TiO2−x  wt% (CuO–V2O5) (1 ≤ x ≤ 2) were milled in ethanol with ZrO2 balls. The mixture powders were dried, granulated with 5 wt% PVA as binder, sieved, and then pressed into pellets (Φ = 11.5 mm × 5.5 mm) under a pressure of 198 MPa. These pellets were sintered from 900 to 1000°C for 5 h in air with a heating rate of 5°C/min.

The bulk densities of the ZTN composite ceramics were measured by Archimedes method. The theoretical densities were calculated by the following formula:

$$\rho_{theo} = \frac{{W_{1} + W_{2} }}{{W_{1} /\rho_{1} + W_{2} /\rho_{2} }}$$
(1)

where ρ 1 , ρ 2 and W1, W2 are the theoretical densities and weight fractions of material 1 and material 2 respectively. The crystal structures were analyzed using X-ray diffraction (XRD) with Cu Ka radiation (Rigaku D/MAX2550, Japan). In order to analyze the microstructure, the sintered specimens were polished and thermally etched at 200°C lower than their sintering temperatures. Microstructures were performed by scanning electron microscope (FEI Co. Eindhoven, Netherlands). The microwave dielectric properties were measured by the TE01δ shielded cavity method using a vector network analysis (ZVB20, Rohde & Schwarz, Germany). The temperature coefficient of resonant frequency (τ f ) was calculated with the following equation:

$$\mathop \tau \nolimits_{f} \left( {ppm/{}^{o}C} \right) = \frac{{f_{80} - f_{25} }}{{f_{25} \times (80 - 25)}} \times 10^{6}$$
(2)

where f80 and f25 were the TE01δ resonant frequency measured at 80 and 25°C, respectively.

3 Results and discussions

Figures 1 and 2 show the XRD patterns of the ZTN specimens sintered at different temperatures and doped with various amount of CV, respectively. For comparison, the XRD pattern of undoped ZTN ceramics sintered at 1100°C is given in Fig. 2. As shown in Figs. 1 and  2, all the samples contain two phases: ZnTiNb2O8 (JCPDS#88-1973) and Zn0.17Nb0.33Ti0.5O2 (ICDD–PDF#39-0291). According to XRD analysis and the literatures [11, 15], the following reaction formulae (3) hold during the sintering process.

$$ZnO + Nb_{2} O_{5} + TiO_{2} \to ZnTiNb_{2} O_{8} ,\;ZnTiNb_{2} O_{8} + TiO_{2} \to Zn_{0.17} Nb_{0.33} Ti_{0.5} O_{2}$$
(3)

The quantitative ratios of ZnTiNb2O8 and Zn0.17Nb0.33Ti0.5O2 phases calculated from the major peak intensities of the two phases are listed in Table 1. As shown in Table 1, for the ZTN specimens doped with 1.5 wt% CV, with increasing sintering temperatures, the Zn0.17Nb0.33Ti0.5O2 phase content increases accompanying with the decreases of ZnTiNb2O8 phase. The content of Zn0.17Nb0.33Ti0.5O2 phase reaches 24.94 % for the ZTN ceramic sintered at 1100 °C. For CV-doped ZTN ceramics sintered at 950 °C, Zn0.17Nb0.33Ti0.5O2 phase content shows the highest value at x = 1.5. Thus, sintering temperature and CV content are two important factors to control the quantitative ratios of ZnTiNb2O8 and Zn0.17Nb0.33Ti0.5O2.

Fig. 1
figure 1

X-ray diffraction patterns of ZTN ceramics added with 1.5 wt% CV sintered at different temperature

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Fig. 2
figure 2

X-ray diffraction patterns of ZTN ceramics added with different amount of CV sintered at 950 °C and the undoped ZTN sintered at 1100 °C

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Table 1 Quantitative ratios between ZnTiNb2O8 and Zn0.17Nb0.33Ti0.5O2 phases, average grain sizes and microwave dielectric properties of ZTN ceramics with different CV content sintered at various temperatures
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Figure 3 shows the SEM micrographs of CV-doped ZTN ceramics after polishing and thermal etching. With increasing sintering temperature and CV content, it can be observed that pores of specimens decrease and the grain sizes increase. Well-dense microstructures are obtained at 950 °C as x ≥ 1.5, indicating that CV is an effective sintering aid to lower the sintering temperature of ZTN ceramics. In order to obtain the average grain sizes, the statistics of average grain size were calculated on the more than 300 particles in each sample, and the mean grain sizes are listed in Table 1. The grain size of the undoped ZTN (Fig. 3d) is in the range of 4–6 μm. The average grain size of the CV-doped specimens are in the range of 1.6–3.2 μm.

Fig. 3
figure 3

SEM micrographs of ZTN ceramics: added with 1.5 wt% CV sintered at a 925 °C; b 950 °C; c 975 °C; d the undoped ZTN ceramic sintered at 1100 °C e added with 1.0 wt% CV sintered at 950 °C, f added with 2.0 wt% CV sintered at 950 °C

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Figure 4 presents the relative density of the CV-doped ZTN ceramics sintered at different temperatures. The dot on the vertical axis shows the relative density of the undoped ZTN ceramic sintered at 1100°C. As shown in Fig. 4, the samples doped with 1.0 wt% CV display the lower relative densities, indicating that 1.0 wt% CV doping is not enough to efficiently densifying the ceramics at these sintering temperatures. Furthermore, the relative densities of samples with 1.5 and 2.0 wt% were more than 95 % at 950°C, which means that the CV is a very effective low-temperature additive.

Fig. 4
figure 4

The relative densities of the ZTN ceramics with the addition of CV sintered at different temperatures

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Figure 5a shows the ε r values of CV-doped ZTN ceramics sintered at different temperatures. The dielectric constant values in Fig. 5a are corrected by following equation [16]:

$$\varepsilon_{r} = \varepsilon_{obs} \left( {1 + 1.5P} \right),\;P = 1 {-} R$$
(4)

where ɛ obs is the measured dielectric constant value, R is the relative density, P is the porosity. The theoretical dielectric constant values of 1.5 wt% CV doped ZTN ceramics calculated by Lichtenecker empirical logarithmic mixing rule [17]:

$$\ln \varepsilon_{r} = \upsilon_{1} \ln \varepsilon_{1} + \upsilon_{2} \ln \varepsilon_{2}$$
(5)

where ε 1 and ε 2 are the dielectric constant values of ZnTiNb2O8 and Zn0.17Nb0.33Ti0.5O2; υ 1and are their volume fraction. The theoretical permittivities given in Fig. 5a are different from the corrected ones, but are close to each other, which indicates that the trend of ε r values of CV-doped ZTN ceramics is depended on sintering temperature and CV content, which is consistent with that of Zn0.17Nb0.33Ti0.5O2 content (ε r  = 95 [18]).

Fig. 5
figure 5

The microwave dielectric properties of CV-doped ZTN ceramics sintered at various temperatures

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Figure 5b illustrates the Q × f values of CV-doped ZTN ceramics sintered at different temperatures. With increasing sintering temperature, the Q × f values initially increase and then decrease thereafter. It is believed that the density play an important role in controlling dielectric loss, as found in other microwave dielectric materials [19–21]. In addition, the decrease of Q × f values at higher temperature(≥950 °C) is due to the increase of the Zn0.17Nb0.33Ti0.5O2 (Q × f = 15,000 GHz [18]) phase content. Thus, both density and the Zn0.17Nb0.33Ti0.5O2 phase content affect the Q × f values of the ceramics. The deterioration in Q × f values of the ZTN specimens with 2.0 wt% CV can be attributed to the presence of excess liquid phases, which may induce more significant deterioration in dielectric properties of ceramics [6].

The τ f values of CV-doped ZTN ceramics sintered at different temperatures are given in Fig. 5c. As known to all, the mixing rule of τ f values can be described as follows:

$$\tau_{f} = \upsilon_{1} \tau_{f1} + \upsilon_{2} \tau_{f2}$$
(6)

where τ f1 and τ f2 represent the τ f values of ZnTiNb2O8 and Zn0.17Nb0.33Ti0.5O2, respectively, υ 1 and are their volume fraction. For 1.5 wt% CV added ZTN ceramics sintered at different temperature, the measured τ f values are close to the theoretical ones. As shown in Table 1, an increase in τ f values consistent with the increase of secondary phase content is observed, which means the secondary phase Zn0.17Nb0.33Ti0.5O2 (τ f  = + 237 ppm/°C [18]) is the dominating factor to control τ f values of ZTN ceramics. Typically, the ZTN ceramics with 1.5 wt% CV and sintered at 950°C have excellent microwave dielectric properties of ε r  = 42.9, Q × f = 12,200 GHz, τ f  = −1.8 ppm/°C.

Figure 6 show the crystalline phases of the 1.5 wt% CV-doped ZTN sample co-fired with 20 wt% Ag powders at 950°C. From the XRD pattern, Ag powders does not produce a new phase in the sample, which means the CV-added ZTN ceramics are able to match with the Ag electrode well.

Fig. 6
figure 6

XRD pattern of 1.5 wt % CV added ZTN sample co-fired with Ag powders at 950°C

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4 Conclusion

ZTN composite ceramics co-doped with CV ceramics could be synthesized by a reaction-sintering process. The sintering behavior, phase compositions and the microwave dielectric properties of the composite ceramics were investigated. The ZTN ceramics with small amounts of CV addition can be well sintered at 950 °C. The change of second phase Zn0.17Nb0.33Ti0.5O2 content depend on CV content and sintering temperature, which ultimately affect the microwave dielectric properties of CV-doped ceramics. The ZTN dielectrics with 1.5 wt% CV sintered at 950 °C showed the optimum microwave dielectric properties: ε r  = 45.9, Q × f = 12,200 GHz, τ f  = −1.8 ppm/ °C. The XRD result of co-fired sample showed that the ZTN ceramics were chemical compatible with Ag at 950 °C.