Enhanced Microwave Dielectric Properties and Sintering Behaviors of Mg₂SiO₄-Li₂TiO₃-LiF Ceramics by Adding CaTiO₃ for LTCC and GPS Antenna Applications

Abstract

Mg₂SiO₄ holds promise for its application in the microwave communication field due to its low dielectric constant and high Q×f value. However, its high negative τ_f and high sintering temperature limit its application in low-temperature co-fired ceramic (LTCC) devices. In this work, Li₂TiO₃ and CaTiO₃ were introduced to improve the τ_f, and LiF was chosen to decrease the sintering temperature. According to XRD patterns and SEM micrographs, the ceramic systems displayed a complex-phase structure, and the microstructure was densified when CaTiO₃ was added. All of the relative densities, dielectric constants, and Q×f values first increased and then decreased as the sintering temperature increased. The MLLC11.5 ceramics sintered at 800 °C could be obtained with the highest Q×f value of 54,581 GHz (at 8.06 GHz), ε_r of 14.13, and τ_f of + 5.81 ppm/°C. Furthermore, it was proven that the MLLC11.5 powders could be co-fired without any reaction with Ag powders at 800 °C, indicating its potential for LTCC application. The MLLC11.5 composition was used to prepare a GPS antenna and showed good prospects for its application in electronic communications.

1. Introduction

As a multilayer ceramic manufacturing technology, low-temperature co-fired ceramic (LTCC) technology enables fabricating multilayer circuit boards and embedding passive devices with negligible packaging costs. Additionally, the devices tend to be smaller, thinner, and lighter due to the three-dimensional package. The conditions of large current and high-temperature resistance can also be satisfied by LTCC technology, contributing to the optimization of the thermal design of electronics. Catering to LTCC application, the ceramics must have a low dielectric constant (ε_r < 15), a high quality factor (Q×f), and a near-zero temperature coefficient of resonant frequency (τ_f ~ ±10 ppm/°C), which can guarantee the LTCC devices have high-frequency and high-speed transmission performance. Therefore, developing ceramics with excellent microwave dielectric properties (MWDPs) and lower sintering temperatures than the melting point of Ag (~961 °C) has become one of the most important topics in the research of microwave dielectric materials.

A lot of low-dielectric materials have been reported, such as Silicates [1,2,3,4], tungstates [5,6,7], germanates [8,9,10,11,12], borates [13], molybdates [14], phosphates [15,16], and some oxides [17,18]. Among them, Mg₂SiO₄, which exhibits excellent MWDPs (ε_r = 6–7, Q×f = 240,000 GHz) [19], is suitable for fabricating LTCC devices. Nevertheless, the negative and large τ_f (~−60 ppm/°C) and the high sintering temperature (≥1350 °C) of Mg₂SiO₄ weaken the frequency stability of the devices and limit their application in the LTCC field. Many studies have reported that the τ_f of Mg₂SiO₄ ceramics can be improved by doping ions [20,21] or adding a compound with a large positive τ_f value, such as TiO₂ [22,23], CaTiO₃ [24], and Ca_0.9Sr_0.1TiO₃ [25], but these compounds generally have a small Q×f value and often decrease the Q×f value of Mg₂SiO₄ ceramics. The poor sintering behavior and decreased Q×f value of Mg₂SiO₄ ceramics can be attributed to the metastable enstatite or protoenstatite (MgSiO₃) secondary phases induced by the conventional solid-state method [26]. To solve these two problems, it has been proven that the phase of MgSiO₃ can be restrained by increasing the content of MgO, leading to an improved Q×f value [26,27]. MgSiO₃ can also be eliminated by a two-step sintering process and mechanical activation [28]. The high sintering temperature of Mg₂SiO₄ ceramics can be lowered by adding an additive with a low melting point, such as LiF [29], Bi₂O₃-Li₂CO₃-H₃BO₃ [24], low-melting-point glass [30,31], or Ba₃(VO₄)₂ [32]. In other words, the MWDPs and sintering behaviors of Mg₂SiO₄ ceramics can be enhanced by adding additives, which often exist in the form of secondary phases.

In this work, Mg₂SiO₄-Li₂TiO₃-LiF ceramics were chosen as the research system, in which Li₂TiO₃ and LiF were introduced to improve the MWDPs and sintering behaviors, respectively, because Li₂TiO₃ has a high Q×f value (~66,000 GHz) and a positive τ_f value (~+22.1 ppm/°C) [33], and LiF has a high Q×f value (~73,880 GHz) and a low melting point (~845 °C) [34]. Moreover, Li₂TiO₃ could form a limited solid solution with LiF, contributing to the densification of the ceramic system. Finally, the composition of (61 wt% Mg₂SiO₄-39 wt% Li₂TiO₃)-8 wt% LiF was selected, and the MWDPs and sintering behaviors were further improved by adding CaTiO₃, and the optimized composition was demonstrated to enable LTCC and GPS antenna application.

2. Materials and Methods

Mg₂SiO₄ (MSO) powders were first synthesized by the conventional solid-state method with the raw materials of MgO (99.9%, Shanghai Macklin Biochemical Co., Ltd., Shanghai, China) and SiO₂ (99.99%, Shanghai Macklin Biochemical Co., Ltd., Shanghai, China) according to the Mg₂SiO₄ stoichiometric ratio calcined at different temperatures (1250 °C, 1300 °C, 1350 °C, and 1400 °C) for 3 h. (61 wt% Mg₂SiO₄-39 wt% Li₂TiO₃)-8 wt% LiF-x wt% CaTiO₃ ceramics (x = 0, 7.5, 8.5, 9.5, 10.5, and 11.5; MLL, MLLC7.5, MLLC8.5, MLLC9.5, MLLC10.5, and MLLC11.5 for short, respectively) were prepared by the conventional solid-state reaction process using the starting materials including the composite Mg₂SiO₄, Li₂TiO₃ (99%, Shanghai Aladdin Biochemical Technology Co., Ltd., Shanghai, China), LiF (99.99%, Shanghai Aladdin Biochemical Technology Co., Ltd., Shanghai, China) and CaTiO₃ (99.5%, Shanghai Aladdin Biochemical Technology Co., Ltd., Shanghai, China). The mixed powders were ball-milled in absolute ethyl alcohol for 24 h. After the slurry was dried, the homogeneous powders were obtained and pressed into pellets, approximately 12 mm in diameter and 6 mm in thickness using 200 MPa pressure and 5% PVA binder. When burning out the binder at 600 °C for 2 h, all the samples were sintered at 775 °C, 800 °C, 825 °C, and 850 °C for 4 h in air using a muffle. To verify LTCC application, 40 wt% Ag powders were mixed with the MLLC11.5 powders without sintering, using the magnetic stirring method in absolute ethyl alcohol for 8 h. After mixing well and drying, the homogeneous powders were sintered at 800 °C for 4 h. The MLLC11.5 green body was coated with silver slurry and then sintered at 800 °C for 4 h. The GPS antenna was designed using HFSS software and prepared using the screen-printing method.

The crystal structures and microstructures of all the samples were examined by X-ray diffraction (XRD, SmartLab 3 KW, Rigaku, Tokyo, Japan) and scanning electron microscopy (SEM, Regulus8100, Hitachi, Tokyo, Japan), respectively. The elemental analysis used energy-dispersive X-ray spectroscopy (EDS) included in the SEM. The volume densities were obtained according to the Archimedes method, and the theoretical densities (D_t) were calculated according to Equation (1)

$$D_t=1/{\displaystyle \sum _{i=1}^n\frac{{\rho }_{wi}}{D_{ti}}},(n\ge 2)$$

(1)

where ρ_wi and D_ti are the weight percentage and theoretical density of the i-th phase, respectively, estimated from the Rietveld refinement results. The relative density is the proportion of the volume density to the theoretical density. ε_r and Q×f are evaluated at about 25 °C by a vector network analyzer (E8363B, Agilent, California, USA). The frequency range is about 8.06 GHz–8.55 GHz. τ_f is calculated according to Equation (2) by measuring the resonant frequencies at 20 °C (f₂₀) and 80 °C (f₈₀). All the error bars were obtained from three groups of data.

$${\tau }_f=\frac{f_{80}-f_{20}}{f_{20}(80-20)}\times {10}^6(\mathrm{ppm}/°\mathrm{C})$$

(2)

3. Results and Discussion

Figure 1 shows the XRD patterns of the MSO powders calcined at different temperatures. As can be seen from Figure 1a, the MSO phase (ICDD 01-074-0714) was discovered in all the samples, combined with the secondary phases identified as MgSiO₃ and MgO, which decreased with the increase in calcining temperature, as shown in Figure 1b,c. The SiO₂ phase was detected when the powders were calcined at 1250 °C, while there was no SiO₂ phase when the powders were calcined at temperatures more than 1250 °C. Thus, it was too troublesome to obtain the composition of the pure MSO phase by the conventional solid-state method, as the MgSiO₃ phase and residual MgO phase always emerged, induced by a small amount of amorphous SiO₂ at high temperatures [26]. When the calcining temperature increased, the residual MgO reacted with MgSiO₃, leading to an increase in the MSO phase [26]. Considering many clumps in the MSO powders calcined at 1400 °C, the optimum calcining temperature was selected as 1350 °C for the MSO powders in this work.

Figure 2a shows the XRD patterns of the MLLC ceramics sintered at 800 °C. A principal crystalline phase identified as Mg₂SiO₄ (ICDD 01-074-0714) was detected for all the samples. The secondary phases could be identified as LiTiO₂, SiO₂, Li₂TiO₃, Li₂MgSiO₄, and CaTiO₃. No LiF phase was discovered from the XRD patterns of the MLLC ceramics, which could be attributed to the solid solution formation between Li₂TiO₃ and LiF [35]. Due to this solid solution, Li⁺ and F⁻ ions entered into Li₂TiO₃ lattice and formed the LiTiO₂ phase, which could also be produced in ceramic systems with an excess MgO content [36,37]. The phases of MgSiO₃, MgO, and amorphous SiO₂ react with Li₂TiO₃, leading to the formation of Li₂MgSiO₄ and LiTiO₂, according to the following reactions:

$$2\mathrm{L}{\mathrm{i}}_2\mathrm{Ti}{\mathrm{O}}_3+\mathrm{MgO}+\mathrm{Si}{\mathrm{O}}_2\to \mathrm{L}{\mathrm{i}}_2\mathrm{MgSi}{\mathrm{O}}_4+2\mathrm{LiTi}{\mathrm{O}}_2+\frac{1}{2}{\mathrm{O}}_2\uparrow$$

(3)

$$2\mathrm{L}{\mathrm{i}}_2\mathrm{Ti}{\mathrm{O}}_3+\mathrm{MgSi}{\mathrm{O}}_3\to \mathrm{L}{\mathrm{i}}_2\mathrm{MgSi}{\mathrm{O}}_4+2\mathrm{LiTi}{\mathrm{O}}_2+\frac{1}{2}{\mathrm{O}}_2\uparrow$$

(4)

As shown in Figure 2a, the CaTiO₃ phase existed in the ceramic systems in the form of the secondary phase without reaction with other phases. Rietveld refinements using Fullprof software were introduced to confirm the weight percentage and the theoretical density of each phase in the MLL ceramic without CaTiO₃, as shown in Figure 2b. The ion occupancy was revised based on the crystal structures of Li₂MgSiO₄, SiO₂, Li₂TiO₃, LiTiO₂, and Mg₂SiO₄. The XRD pattern of the MLL ceramic displayed good refinement. The refinement result is shown in

Table 1. Rietveld refinement result of the MLL ceramic.

Phase	Dt (g/cm3)	Vt (×106 pm3)	ρt (wt%)	Rwp (%)	Rp (%)	χ2	D (g/cm3)
Mg2SiO4	3.311	290.337	59.07	13.9	9.82	5.42	3.381
LiTiO2	4.068	70.890	24.18
Li2TiO3	3.404	428.252	8.60
Li2MgSiO4	2.575	336.059	6.27
SiO2	2.270	175.838	1.88

R_wp: reliability factor of weighted patterns; R_p: reliability factor of patterns; and χ²: goodness of fit.

. The theoretical density of the MLL ceramic was calculated according to Equation (1), with a value of about 3.381 g/cm³. As CaTiO₃ could not react with the MLL, the theoretical densities of the MLLC ceramics could also be calculated according to Equation (1).

Figure 3 shows the SEM micrographs of the cross-sections of the MLLC ceramics sintered at 800 °C. No grain changes were discovered for any of the samples when increasing the CaTiO₃ content. Some pores were detected for the MLL ceramic, and a dense microstructure was detected for all the samples with CaTiO₃ additives, which indicated that CaTiO₃ additives could promote the densification of the MLLC ceramics. Figure 4 shows the SEM micrographs of the cross-sections of the MLLC9.5 ceramics sintered at different temperatures. The micrograph differences between Figure 4a–d could be due to the breaking of the ceramics, leading to different flatnesses. The grain size of the MLLC9.5 ceramic increased with the increase in the sintering temperature, and some pores were observed in all the samples except for those sintered at 800 °C, indicating that 800 °C was the optimal sintering temperature of the MLLC ceramics.

Figure 5 shows the relative densities, volume densities, dielectric constants, Q×fs, and τ_fs of the MLLC ceramics. All the relative densities, volume densities, dielectric constants, and Q×f values first increased and then decreased with the increase in the sintering temperature and achieved a maximum value in the samples sintered at 800 °C. When the sintering temperatures were higher than 800 °C, the air in the ceramics could not escape in time, and isolated pores remained trapped, resulting in higher porosity [38]. The dielectric constants and Q×f values changed with the relative densities, which indicated that porosity played an important role in the MWDPs. The dielectric constants also varied in the additive amount of CaTiO₃ content due to the high dielectric constant of CaTiO₃ ceramics (~162) [39]. After adding the CaTiO₃ additives, an obvious increase in the relative density, dielectric constant, and Q×f was observed for all the MLLC ceramics, revealing that the MWDPs were improved with the decrease in porosity. The τ_fs of the MLLC ceramics sintered at 800 °C increased linearly with the increase in the CaTiO₃ content. The variation of τ_fs could mainly be attributed to the CaTiO₃ content and described according to the Lichtenecker empirical rule

$${\tau }_f=V_1{\tau }_{f1}+V_2{\tau }_{f2}$$

(5)

where V₁ and V₂ are the volume fractions and τ_f1 and τ_f2 are the τ_f value of each phase. The increase in the τ_f value could be due to the increased concentration of the CaTiO₃ phase, which was reported to have a high positive τ_f value of +859 ppm/°C [39]. A near-zero τ_f value of +5.81 ppm/°C was obtained for the MLLC11.5 ceramics sintered at 800 °C with an optimal Q×f value of 54,581 GHz (at 8.06 GHz) and a low ε_r value of 14.13.

As seen from the XRD pattern of MLLC11.5-40wt%Ag powders sintered at 800 °C in Figure 6a, Ag powders could not react with MLLC11.5 powders and mainly existed in the form of the secondary phase. Figure 6b shows a SEM micrograph of the cross-section of the MLLC11.5 ceramic, whose surface was coated with silver slurry. The ceramic was co-fired at 800 °C for 4 h and displayed a clear interface between the solidified silver slurry and the ceramic, marked as a and b, respectively. Figure 6c,d show the elemental analysis results of areas a and b by EDS, which displayed that areas a and b belonged to Ag and the MLLC11.5 ceramic, respectively. This result indicates that MLLC11.5 ceramics could be suitable for LTCC application.

The MLLC11.5 composition was also employed to prepare a GPS antenna composed of a ground plane, radiation layer, and dielectric substrate. The designed sample is shown in Figure 7a. The dielectric substrate was the MLLC11.5 ceramic, about 1.5 mm thick and 21 mm in diameter. The radiation layer was a camber line with a 1 mm width, whose length could be modified to satisfy the GPS frequency (~1.575 GHz). Another part of the front face was the ground plane, which also included a part of the side face and half of the back face of the dielectric substrate. Considering the size limit of the dielectric substrate, the radiation layer could connect with the ground plane through a metal wire with 50 Ω impedance, which could increase the inductance of the antenna to reduce the resonant frequency. The GPS antenna was designed with HFSS software and prepared with the screen-printing process according to the designed size. The front and back face of the GPS antenna is shown in Figure 7b,c, respectively. The metal pattern of the side face was handmade. The soldering was guaranteed by thickening and widening the silver slurry of the solder joint position. Figure 7d shows the return loss (S11) of the GPS antenna. Both the simulated and experimental data of the S11 were less than −10 dB at the resonant frequency. By modifying the length of the radiation layer, the resonant frequency was improved and nearly identical to the simulated data. The GPS signal test was carried out for the antenna by using a hardware system and displayed in the software, as shown in Figure 7e. A total of 11 or 12 Pcs GPS satellites were detected at the same time and displayed more than a 28 dBHz carrier-to-noise ratio (C/N0) on average, indicating antenna application [40].

4. Conclusions

MLLC ceramics with different amounts of CaTiO₃ additives were prepared using the conventional solid-state method. The effects of CaTiO₃ contents on the structures, sintering behaviors, and MWDPs were studied. A mixed-phase structure composed of Mg₂SiO₄, LiTiO₂, Li₂TiO₃, Li₂MgSiO₄, SiO₂, and CaTiO₃ was detected using the XRD pattern. The relative density significantly increased after adding CaTiO₃, leading to a significantly increased Q×f. The dielectric constant and Q×f first increased and then decreased with the increase in the sintering temperature, and achieved a maximum value when the MLLC11.5 ceramics were sintered at 800 °C with an ε_r of 14.13 and Q×f of 54,581 GHz (at 8.06 GHz). The τ_f increased linearly with the increase in the CaTiO₃ content and reached three near-zero values of −5, +2.59, and +5.81 ppm/°C for MLLC9.5, MLLC10.5, and MLLC11.5 compositions, respectively. The MLLC11.5 composition could be co-fired without any reaction with Ag powders at 800 °C, indicating LTCC application. In the investigation of the GPS antenna, the MLLC11.5 composition exhibited good potential for antenna application.