Phase compositions, microstructures, and microwave dielectric properties of Li₂Zn₃Ti₄O₁₂-based temperature stable materials modified by CaTiO₃ additions

Abstract

In present work, the microwave dielectric properties of CaTiO₃-added Li₂Zn₃Ti₄O₁₂ ceramics prepared by the conventional solid-state method have been investigated. The phase composition, microstructure and microwave dielectric properties of (1 − x) Li₂Zn₃Ti₄O₁₂–x CaTiO₃ (x = 3, 4, 5, 6 and 7 wt%) ceramics were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), EDS and network analyzer. The XRD and EDS results indicated that the ceramics involved Li₂Zn₃Ti₄O₁₂ and CaTiO₃ phases. The SEM micrographs shown that as the CaTiO₃ contents increased, the average grain size of the composition ceramics decreased. The microwave dielectric properties of the composition ceramics can be effectively controlled by varying the CaTiO₃ contents. The values of dielectric constant (ε_r) and τ_f of the composition ceramics were increased, and the quality factor (Q × f) was decreased with increasing the CaTiO₃ contents. Typically, a new temperature stable microwave dielectric material of 0.94 Li₂Zn₃Ti₄O₁₂–0.06 CaTiO₃ with excellent microwave dielectric properties of ε_r = 21.7, Q × f = 61,490 GHz, and τ_f = + 2.68 ppm/°C was attained when sintered at 1175 °C for 4 h.

1 Introduction

The recent rapid advances in Internet of Things, the Industrial Internet, electronic warfare, intelligent transport systems and the fifth generation wireless systems. Microwave dielectric ceramics, as a key material used in these applications, have been received much more attention. It is known that miniaturization, integration, and high reliability are the important tendency in today’s microwave electronic devices and portable terminals. Therefore, materials with high dielectric constant (ε_r ˃ 10), high quality factor (Q × f ≥ 10,000 GHz) and near-zero temperature coefficient of resonant frequency (|τ_f| ≤ 10 ppm/°C) are strongly required. Especially, for the operating frequency in nowadays are increasing (e. g: Millimeter wave or sub-millimeter wave), materials with near-zero τ_f value become more and more critical [1,2,3,4].

In recent years, a great number of Li-containing compounds microwave dielectric ceramics with spinel-structure such as Li₂MTi₃O₈ (M = Mg, Zn, Co) [5, 6], Li₂M₃Ti₄O₁₂ (M = Zn, Mg, Co) [7,8,9] and Li₂ZnTi₅O₁₂ [10] etc., have been extensively studied due to their excellent microwave dielectric properties. Among in these ceramics, the Li₂Zn₃Ti₄O₁₂ ceramic based on Li₂O–ZnO–TiO₂ ternary systems have attracted much attention for low cost, light weight, eco-friendly of the raw materials and excellent microwave dielectric properties: ε_r = 20.6, Q × f = 106,700 GHz and τ_f = − 48 ppm/°C when sintered at 1075 °C for 2 h [7]. Unfortunately, the negative τ_f value (τ_f = − 48 ppm/°C) prohibits its practical applications. Therefore, compensate the τ_f value near-zero simultaneously without much deteriorating the microwave dielectric properties is urgent. To achieve a near-zero τ_f for the Li₂Zn₃Ti₄O₁₂ ceramic, up till now, introducing a new material with an opposite τ_f value in the ceramic system has been demonstrated as an effective approach to adjust the τ_f value near-zero [11, 12]. Summarize previous works, CaTiO₃ have often been utilized as a τ_f compensation materials due to it’s high positive τ_f (τ_f ~ + 800 ppm/°C) value and do not easily react at high temperature [13,14,15,16,17].

In this work, CaTiO₃ as a τ_f compensation material and mixture with Li₂Zn₃Ti₄O₁₂ ceramic to form the (1 − x) Li₂Zn₃Ti₄O₁₂–x CaTiO₃ ceramics system to achieve the τ_f value of Li₂Zn₃Ti₄O₁₂ ceramic near-zero. In addition, the phases, sintering behaviors, and microwave dielectric properties of (1-x) Li₂Zn₃Ti₄O₁₂–x CaTiO₃ ceramics were investigated systematically.

2 Experimental

The Li₂Zn₃Ti₄O₁₂ and CaTiO₃ powders were synthesized via the solid-state reaction route using analytical grade powders of Li₂CO₃ (AR, ≥ 99%, GuoYao Co. Ltd., Shanghai, China), ZnO (AR, ≥ 99%, Macklin Co. Ltd., Shanghai, China), TiO₂ (AR, ≥ 99%, Macklin Co. Ltd., Shanghai, China) and CaCO₃ (AR, ≥ 99%, Macklin Co. Ltd., Shanghai, China) as raw materials. Stoichiometric Li₂CO₃, ZnO, TiO₂ and CaCO₃ were mixed according to the formula of Li₂Zn₃Ti₄O₁₂ and CaTiO₃ with ZrO₂ balls by ball mill with ethanol for 4 h, respectively. The mixtures were dried and calcined at 1000 °C for 4 h to form Li₂Zn₃Ti₄O₁₂ and CaTiO₃ phase. Thereafter, The calcined powders were mixed according to the desired composition (1 − x) Li₂Zn₃Ti₄O₁₂–x CaTiO₃ (x = 3, 4, 5, 6 and 7 wt%) were mixed together and reground in ethanol medium for 4 h to get homogeneously mixture powders. After drying, the mixed powders were ground with 5 wt% polyvinyl alcohol (PVA) solution and then pelleted to 15 mm diameter and 7–8 mm thick disks at 5 MPa by hydraulic pressing. The thick disks samples were heated at 550 °C for 2 h to remove the organic binder and then sintered at 1100–1200 °C for 4 h in air at a heating rate of 5 °C/min.

The phases composition were identified by X-ray diffractometer (PANalytical Empyrean Series 2, UK) using CuKá radiation. The microstructures of sintered surfaces were performed by scanning electron microscope (SEM, JSM-6460LV, and Japan) coupled with energy-dispersive X-ray spectrometer (EDS, Philips). The bulk densities of the samples were measured by the Archimedes method. The ε_r and Q values were measured in the TE011 mode by using the Hakki-Coleman dielectric resonator method using a network analyzer (HP83752A, the United States) in a wide frequency (1–20 GHz).The temperature coefficients of resonant frequency (τ_f) were measured with changing temperatures from 20 to 60 °C defined as follows:

$$\tau_{f} = \left\{ { \, (f_{ 60} - f_{ 20} ) /\left( {f_{ 20} \times { 4}0} \right) \, } \right\} \times 10^{ 6} \left( {{\text{ppm}}/^\circ {\text{C}}} \right)$$

where f₂₀ and f₆₀ represent the resonant frequency at 20 and 60 °C, respectively.

3 Results and discussion

The XRD results of 0.94 Li₂Zn₃Ti₄O₁₂–0.06 CaTiO₃ ceramics sintered at 1100–1200 °C for 4 h are given in Fig. 1. The peaks of the five-group samples indicating there are two phases co-exist in the 0.94 Li₂Zn₃Ti₄O₁₂–0.06 CaTiO₃ ceramics system, which including Li₂Zn₃Ti₄O₁₂ as the main crystalline phase (PDF # 44-10381) and CaTiO₃ as a minor phase (PDF # 42-0423), no other phases are detected. With the sintering temperatures increase from 1100 to 1200 °C, the peaks intensity changed not significantly. Figure 2 shows the XRD data of (1 − x) Li₂Zn₃Ti₄O₁₂–x CaTiO₃ (3 wt% ≤ x ≤ 7 wt%) ceramics sintered at 1175 °C for 4 h. The peaks intensity identified gradually increased as x varies from 3 to 7 wt%, which corresponding to the increase of CaTiO₃ mass fraction in the mixture. These results indicated that the CaTiO₃ not only has a good compatibility with Li₂Zn₃Ti₄O₁₂ ceramic but also possible to adjust the τ_f value of Li₂Zn₃Ti₄O₁₂ ceramic near zero.

Fig. 1

X-ray diffraction patterns of 0.94 Li₂Zn₃Ti₄O₁₂–0.06 CaTiO₃ ceramics sintered at different temperatures for 4 h

Fig. 2

X-ray diffraction patterns of (1 − x) Li₂Zn₃Ti₄O₁₂–x CaTiO₃ ceramics sintered at 1175 °C for 4 h with different x values

The typical SEM micrographs of 0.94 Li₂Zn₃Ti₄O₁₂–0.06 CaTiO₃ ceramics sintered at 1100–1200 °C for 4 h are displayed in Fig. 3. All of the micrographs show two types grain. As shown in Fig. 3a–c, with the sintering temperature increased, the microstructures become more compact and the amount of pores decreases when sintered at 1125 °C to 1150 °C, but not full-dense. As sintering temperature reaches 1175 °C, a homogeneous and dense microstructure is obtained (Fig. 3d). However, abnormal grain growth is observed at temperatures increases to 1200 °C. The SEM photographs of (1 − x) Li₂Zn₃Ti₄O₁₂–x CaTiO₃ (3 wt% ≤ x ≤ 7 wt%) ceramics sintered at 1175 °C for 4 h are illustrated in Fig. 4. All photograph shows a homogeneous and compact microstructures, and the average grain size decreased as x increased from 3 to 7 wt%.

Fig. 3

SEM micrographs of (1 − x) Li₂Zn₃Ti₄O₁₂–x CaTiO₃ ceramics sintered for 4 h at different temperatures: a 1100 °C, b 1125 °C, c 1150 °C, d 1175 °C, and e 1200 °C

Fig. 4

SEM micrographs of (1 − x) Li₂Zn₃Ti₄O₁₂–x CaTiO₃ ceramics sintered at 1175 °C for 4 h: ax = 3 wt%, bx = 4 wt%, cx = 5 wt%, dx = 6 wt% and ex = 7 wt%

To further understand the crystal of the two grains mentioned in Fig. 3. The EDS results of 0.94 Li₂Zn₃Ti₄O₁₂–0.06 CaTiO₃ ceramics sintered at 1175 °C for 4 h are shown in Fig. 5. Grains with different shape and size are labeled as “spectrum 1” and “spectrum 2”. From the EDS results, the “spectrum 1” grain are rich in Ca and Ti but poor in Zn, the “spectrum 2” grain are rich in Zn and Ti but poor in Ca. Combined with the XRD results shown in Fig. 1 and Fig. 2, the small cubic-shaped grains are CaTiO₃ phase and the large grains are Li₂Zn₃Ti₄O₁₂ phase.

Fig. 5

The EDS results for the 0.94 Li₂Zn₃Ti₄O₁₂–0.06 CaTiO₃ ceramics sintered at 1175 °C for 4 h

Figure 6 shows the bulk densities of (1 − x) Li₂Zn₃Ti₄O₁₂–x CaTiO₃ (3 wt% ≤ x ≤ 7 wt%) ceramics sintered at 1100–1200 °C for 4 h. With increasing sintering temperature, the bulk density of all sintered samples shown a trend of increasing during the sintering temperature rang from 1100 to 1175 °C, and reaching a maximum at 1175 °C. However, a slight decrease in density as the sintering temperature reaches 1200 °C, which possibly owing to the abnormal grain growth. Obviously, as the CaTiO₃ content increased, the bulk density of (1 − x) Li₂Zn₃Ti₄O₁₂–x CaTiO₃ (3 wt% ≤ x ≤ 7 wt%) ceramics gradually decreases. The main reason may attributable to the difference in the theoretical density of Li₂Zn₃Ti₄O₁₂ (ρ_th = 4.3 g/cm³) and CaTiO₃ (ρ_th = 3.98 g/cm³).

Fig. 6

Variation in the bulk density of (1 − x) Li₂Zn₃Ti₄O₁₂–x CaTiO₃ (3 wt% ≤ x ≤ 7 wt%) ceramics as a function of the sintering temperature

The ε_r of (1 − x) Li₂Zn₃Ti₄O₁₂–x CaTiO₃ (3 wt% ≤ x ≤ 7 wt%) ceramics sintered at 1100–1200 °C for 4 h are illustrated in Fig. 7. A same trend of ε_r and bulk density with sintering temperature is observed. With sintering temperature increasing from 1100 to 1175 °C, the ε_r values increased and reached a maximum at 1175 °C. However, when sintering temperature reaches 1200 °C, the ε_r values decreased. Furthermore, for the same temperature, the ε_r values of (1 − x) Li₂Zn₃Ti₄O₁₂–x CaTiO₃ (3 wt% ≤ x ≤ 7 wt%) ceramics increased from 19.1 to 22.3 as the CaTiO₃ increased from 3 to 7 wt%, which due to CaTiO₃ has an ε_r (ε_r ~ 170) substantially greater than that of Li₂Zn₃Ti₄O₁₂ (ε_r ~ 20.6). Summary, the ε_r of (1 − x) Li₂Zn₃Ti₄O₁₂–x CaTiO₃ (3 wt% ≤ x ≤ 7 wt%) ceramics affected not only by the sintering temperature but also by the CaTiO₃ contents.

Fig. 7

Variation in the dielectric constant of the (1 − x) Li₂Zn₃Ti₄O₁₂–x CaTiO₃ (3 wt% ≤ x ≤ 7 wt%) ceramics as a function of sintering temperature

The Q × f values of (1 − x) Li₂Zn₃Ti₄O₁₂–x CaTiO₃ (3 wt% ≤ x ≤ 7 wt%) ceramics sintered at 1100–1200 °C for 4 h are presented in Fig. 8. As the sintering temperature increases, the Q × f values of the ceramics increases first and reached a maximum at 1175 °C. While the sintering temperature reaches 1200 °C, the Q × f values decreased. The trend of the Q × f values of (1 − x) Li₂Zn₃Ti₄O₁₂–x CaTiO₃ (3 wt% ≤ x ≤ 7 wt%) ceramics are match well with bulk density values. As known, the Q × f values are not only affected by the lattice vibrational modes but also affects by the second phase, pores, impurities, and density etc. extrinsic factors [18, 19]. While for the same temperature, as the x increases from 3 to 7 wt%, the Q × f values of the ceramics decreases, which owing to the large difference in the Q × f values of Li₂Zn₃Ti₄O₁₂ (Q × f ~ 106,700 GHz) and CaTiO₃ (Q × f ~ 3600 GHz). In this works, therefor, densification and second phase are plays important roles in controlling the the Q × f values of (1 − x) Li₂Zn₃Ti₄O₁₂–x CaTiO₃ (3 wt% ≤ x ≤ 7 wt%) ceramics. A maximum Q × f value of 61,490 GHz could be achieved for the 0.94 Li₂Zn₃Ti₄O₁₂–0.06 CaTiO₃ ceramic sintered at 1175 °C for 4 h.

Fig. 8

Variation in the Q × f values of (1 − x) Li₂Zn₃Ti₄O₁₂–x CaTiO₃ (3 wt% ≤ x ≤ 7 wt%) ceramics as a function of the sintering temperature

Figure 9 illustrates the τ_f values of the (1 − x) Li₂Zn₃Ti₄O₁₂–x CaTiO₃ (3 wt% ≤ x ≤ 7 wt%) ceramics sintered at 1175 °C for 4 h. As shown, with the x variation from 3 to 7 wt%, the τ_f values increased from − 31.2 ppm/°C to + 11.6 ppm/°C. Generally speaking, the composition, additives, and second phases are the main factors for the τ_f values of microwave dielectric ceramics [20]. By increasing x, the τ_f values of (1 − x) Li₂Zn₃Ti₄O₁₂–x CaTiO₃ (3 wt% ≤ x ≤ 7 wt%) ceramics varied toward positive direction. In addition, according to the mixing rules, the τ_f values were compensated. Therefor, a near-zero τ_f values of (1 − x) Li₂Zn₃Ti₄O₁₂–x CaTiO₃ (3 wt% ≤ x ≤ 7 wt%) ceramics could be obtained by doping an appropriate content of CaTiO₃. When the CaTiO₃ amount is 6 wt%, a near-zero τ_f value with + 2.68 ppm/°C obtained.

Fig. 9

The τ_f values of (1 − x) Li₂Zn₃Ti₄O₁₂–x CaTiO₃ (3 wt% ≤ x ≤ 7 wt%) ceramics sintered at 1175 °C for 4 h

4 Conclusions

In this work, CaTiO₃ as a τ_f compensation material added to Li₂Zn₃Ti₄O₁₂ ceramic to form a new ceramics systems of (1 − x)–x CaTiO₃ (3 wt% ≤ x ≤ 7 wt%) were investigated systematic. The XRD and EDS results demonstrated that the (1 − x) Li₂Zn₃Ti₄O₁₂–x CaTiO₃ (3 wt% ≤ x ≤ 7 wt%) compositions ceramics shown a mixture of two phases, Li₂Zn₃Ti₄O₁₂ and CaTiO₃ phase. The SEM micrographs shown that with the CaTiO₃ increased, the average grain size of (1 − x) Li₂Zn₃Ti₄O₁₂–x CaTiO₃ (3 wt% ≤ x ≤ 7 wt%) composition ceramics decreased. Moreover, the microwave dielectric properties of the composition ceramics can be effectively controlled by varying the CaTiO₃ contents. The values of the ε_r and τ_f were found to increase, but the Q × f values were found to decrease when the CaTiO₃ contents increased from 3 to 7 wt%. Typically, a new temperature stable microwave dielectric material of 0.94 Li₂Zn₃Ti₄O₁₂–0.06 CaTiO₃ with excellent microwave dielectric properties of ε_r = 21.7, Q × f = 61,490 GHz, and τ_f = + 2.68 ppm/°C were obtained when sintered at 1175 °C for 4 h.