Effects of Cu substitution on microstructures and microwave dielectric properties of Li2ZnSiO4 ceramics

Herein, the influence of Cu2+ substitution on the phase composition, bulk density, microstructures, and microwave dielectric properties of Li2CuxZn1−xSiO4 (0 ≤ x ≤ 0.06) ceramics prepared by a solid-state reaction were investigated. The results of XRD and mapping showed that Cu2+ substitution can avoid the influence of secondary phase on the properties of samples. According to the analysis of bulk density, microstructure and microwave dielectric properties, a proper amount of Cu substitution not only improved the sintering characteristics of Li2CuxZn1−xSiO4 ceramics, reduced the densification temperature from 1250 °C to 950 °C, but also increased the Q×f value. Furthermore, Cu2+ substitution also improved the temperature stability of the samples. Particularly, the Li2Cu0.04Zn0.96SiO4 ceramics sintered at 950 °C for 5 h possessed excellent microwave dielectric properties: εr = 5.624, Q×f = 12,764 GHz, and τf = −77 ppm/°C, exhibiting a potential for the low temperature co-fired ceramic applications.


Introduction
In recent years, the continuous innovation and development of communication technology has brought great convenience to all aspects of human production and social activities. Microwave dielectric ceramic is a kind of dielectric ceramic material which is used in microwave band circuit and realizes specific functions. It has become the key basic material of modern communication technology and the cornerstone of communication technology innovation. Microwave dielectric ceramics have three important parameters: (A) dielectric constant (εr), which is related to the delay of microwave propagation in materials (Td = L√εr/c, where L is the traveling distance of the signal and c is the speed of light) [1]. Low dielectric constant corresponds to short delay time, it is conducive to signal propagation. (B) quality factor (Q×f), which represents the dielectric loss of the material and is related to the reliability and life of the material. (C) the temperature coefficient of the resonant frequency (τf), the closer τf value is to zero, the better the temperature stability of the resonant frequency of the sample. Low temperature co-fired ceramic (LTCC) is a technology used to package passive and active components into multi-layer structures to achieve miniaturization and multi-function. In addition to the above three parameters, it should also meet the requirements of co-firing with silver electrode. Therefore, the sintering temperature of LTCC materials should be lower than the melting point of Ag (961℃) [2,3].
Silicate is a typical microwave dielectric ceramic with low dielectric constant, but its high temperature sintering characteristics limit its application in LTCC devices. Compared with other silicate systems, Li2ZnSiO4 and Li2MgSiO4 ceramics have lower sintering temperature (ST=1250 ℃ ) [4,5], which is more conducive to the research of low temperature modification. The research on low temperature sintering of Li2MgSiO4 has been reported, but the low temperature characteristics of Li2ZnSiO4 have not been systematically studied. Generally, there are three methods to reduce the sintering temperature of ceramics: using superfine raw materials, adding sintering additives or ion substitution. The high cost of using superfine raw materials is not conducive to industrial mass production. Adding sintering additives is a common method to reduce sintering temperature, but it is easy to introduce impurities or increase dielectric loss. Compared with the former two methods, the substitution of ions with similar radius and lower sintering temperature can avoid the formation of secondary phase and realize low temperature sintering.
According to the previous research, Cu 2+ can reduce the sintering temperature to a certain extent [6,7], and the radius of Cu 2+ is close to that of Zn 2+ [8]. Therefore, Cu 2+ ions are used to replace Zn 2+ ions to adjust the sintering characteristics of Li2ZnSiO4 ceramics to achieve low-temperature sintering. In addition, the effects of Cu substitution on the phase, microstructures and microwave dielectric properties of the samples were analyzed by XRD and scanning electron microscopy (SEM).

Experimental and methods
The Li2CuxZn1−xSiO4 (0 ≤ x ≤ 0.06) ceramics were fabricated using the solid-state reaction process. High-purity starting materials of Li2CO3(99%), CuO (99%), ZnO (99%), and SiO2 (99%) were weighed according to the desired molar ratio. The raw materials were mixed and ball-milled for 6 h in a nylon container, using distilled water and ZrO2 balls as the grinding medium. The processed powders were calcined at 925°C in an alumina crucible for 5 h. The calcined powders were re-milled and then dried. After that, the powders with 12 wt% PVA as a binder were pressed into pellets with 12 mm-diameter and about 6 mm height under a pressure of 10 MPa. All pellets sintered at 925, 950, 975 and 1000°C for 5 h, respectively.
The crystal phases of the ceramics were characterized by X-ray diffraction analysis with Cu Kα radiation. The microstructure, energy-dispersive spectra (EDS), back-scattered electron (BSE), and mapping images were observed via scanning electron microscopy. The bulk density of the samples was measured using an Archimedes method. Microwave dielectric properties were measured through a Hakki-Coleman dielectric resonator method and a cavity method. The τf values were measured in the temperature ranging from 25 °C to 85°C based on the following equation: where f1 and f2 represent the resonant frequencies at temperatures 25°C and 85 °C respectively.

Results and discussions
The bulk densities of Li2CuxZn1−xSiO4 (0 ≤x≤ 0.06) ceramics sintered at different temperatures are shown in Fig.1. It is obvious that with the increase of Cu 2+ substitution, the bulk densities of the samples increase first and then decrease, which indicates that the bulk densities depend strongly on the content of Cu. When the sintering temperature increases from 925°C to 1000°C, the bulk density values of all specimens increase approaching to a maximum value at 950 °C, indicating that 950 °C is the optimal temperature of the Li2CuxZn1−xSiO4 ceramics. Therefore, we select the samples sintered at 950 °C for further investigation in detail. The XRD patterns of Li2CuxZn1−xSiO4 (0 ≤x≤ 0.06) ceramics sintered at 950 ℃ for 5 h are shown in Fig.2. The patterns of all the samples match well with the monoclinic Li2ZnSiO4 phase (ICSD#8237), and no secondary phase existed. Obviously, no phase containing Cu 2+ was observed, indicating that Cu 2+ entered the lattice and formed a solid solution in Li2CuxZn1−xSiO4ceramics, which is mainly due to the similar ionic radius of Cu 2+ (0.73 Å, CN=6) and Zn 2+ (0.74 Å, CN=6) [8]. The microstructure of Li2CuxZn1−xSiO4 (0 ≤x≤ 0.06) ceramics sintered at 950 ℃ for 5 h was characterized by SEM, as shown in Fig. 3 (a-d). Obviously, the grains of the samples not substituted by Cu 2+ are small and heterogeneous, accompanied by the existence of pores. With the increase of substitution amount, the grain size increases and the porosity decreases. When x =0.04, dense and uniform microstructure is obtained. With the further increase in x value, the grain boundary blurs and the pores reappear, which indicates that excessive substitution is not conducive to the further densification of the samples. The analysis of BSE and EDS surface scanning of Li2Cu0.04Zn0.96SiO4 ceramics is shown in Fig. 3 (e). It can be observed that there is only one kind of grain in the sample, and the element content ratio is close to Li2Cu0.04Zn0.96SiO4. In addition, the mapping image shows that the distribution of Cu is similar to that of Zn. These results further prove that Cu 2+ substitution does not produce secondary phase and forms a solid solution [9], which is consistent with XRD results. The relative permittivity of Li2CuxZn1−xSiO4 (0 ≤x≤ 0.06) ceramics sintered at different temperatures is present in Fig.4(a). The εr values of the samples at different sintering temperatures present a similar trend, which first increases and then decreases with the increase of Cu substitution. Generally, the εr value of microwave ceramics is affected by both external factors (bulk density and secondary phase) and internal factors (molecular polarizability) [3]. There is no secondary phase in this study, so the effect of secondary phase can be ignored. According to Fig. 4(a) and Fig. 1, the dielectric constant and bulk density show the same trend at each temperature, indicating that bulk density as an external factor has an obvious effect on the εr value of Li2CuxZn1−xSiO4 ceramics. In addition, the molecular polarizability is also one of the factors that affect the εr value. Because the ion polarizability of Cu 2+ (2.11Å 3 ) is higher than that of Zn 2+ (2.01 Å 3 ) [10], the molecular polarizability increases with the increase of Cu substitution, which result in the initial increase of the εr value.
The variations in Q × f value of Li2CuxZn1−xSiO4 (0 ≤x≤ 0.06) ceramics sintered at different temperatures are display in Fig.4(b). As we all know, the Q × f value is closely related to the dielectric loss, which can be separated into intrinsic loss and extrinsic loss [11]. In the study, the extrinsic loss mainly includes microstructures and bulk density. As shown in Fig.4(b), with the increase of Cu substitution, Q × f value increases first and then decreases. When x = 0.04 and sintering temperature is 950 ℃, the optimal value is obtained, which is consistent with the bulk density and indicates that the influence of bulk density on the Q × f value can be not neglected. Furthermore, the microstructures present that the samples sintered at 950 ℃ are densified with the increase in x value. When x = 0.04, the sample has the most uniform and dense microstructure, which has the least defects, the lowest porosity and the smallest external loss, result in the optimal Q × f value. The subsequent decrease of Q × f value is due to the over burn caused by excessive substitution, which increases the porosity, decreases the bulk density and increases the dielectric loss.
The τf value is a parameter representing the temperature stability about resonance frequency of microwave dielectric ceramic. The closer it is to zero, the better the temperature stability. The τf values of Li2CuxZn1−xSiO4 (0 ≤x≤ 0.06) ceramics sintered at 950 ℃ are shown in Fig.4(c). It is gratifying that with the increase of x value, the τf value increase gradually, indicating that Cu substitution optimizes the temperature stability of the sample to a certain extent. According to the research of Lai et al. [7], the change of temperature coefficient of resonant frequency may be caused by the substitution of Cu 2+ for Zn 2+ , which affects the interaction between cation and anion, resulting in the change of bond length, structural order and polyhedral properties.