DOI:10.30919/esmm5f603

Received: 04 May 2019
Revised: 04 Dec 2019
Accepted: 08 Dec 2019
Published online: 09 Dec 2019

Sintering Behavior and Microwave Dielectric Properties of Low-Loss Li6Mg7Zr3O16 Ceramics Doped with Different LiF Additives

H.R. Tian1, C.F. Xing1, H.T. Wu1* and Z.H. Wang2*

1School of Materials Science and Engineering, University of Jinan, Jinan 250022, China

2School of Materials Science and Engineering, Qilu University of Technology (Shandong Academy of Sciences), Jinan, 250353, China

Abstract

The Li6Mg7Zr3O16 samples were prepared with the pure cubic phase at sintering temperatures of 1300-1550 °C using the traditional solid state method in our previous work. Sintering characteristics and microwave properties were investigated as a function of sintering temperatures. The samples sintered at 1500 °C showed the best properties of the Q·f value of 81,284 GHz (at 8.94 GHz), dielectric constant value of 14.22 and the τf value of -21.56 ppm/°C. Now complex permittivity value of Li6Mg7Zr3O16 ceramic sintered at 1500 °C were characterized by the infrared spectra based on the classical harmonic oscillator model. In order to reduce the sintering temperature, lithium fluoride were used as sintering additives, and then the apparent densities, phase compositions and dielectric properties of Li6Mg7Zr3O16 -x (0 wt% LiF ≤ x ≤ 5 wt% LiF) were discussed as a function of lithium fluoride additions. As a result the densification temperatures for Li6Mg7Zr3O16-5 wt% LiF were reduced to be 1100 °C, which were significantly lower than that of the matrix (1500 °C). Excellent microwave dielectric properties were obtained in Li6Mg7Zr3O16-5 wt% LiF ceramics sintered at 1100 °C with εr=13.67, Q·f = 132,600 GHz (at 9.26 GHz) and τf = -18.89 ppm/°C.


Table of Content

Our work provided a facile strategy for lowing the sintering temperature of Li6Mg7Zr3O16 by adding appropriate amount of LiF additions.

 

 

 

 

 

 


Keywords: Low-temperature sintering; Li6Mg7Zr3O16 ceramics; Infrared spectra; Microwave dielectric properties; LiF addition


1. Introduction

With the rapid development of the wireless communication industry, microwave dielectric ceramics with high performances have been widely investigated and used for microwave components, such as filters, antennas, oscillators and resonators.1-4 To satisfy the specific requirements of current and future microwave devices, high-performance ceramics with an appropriate dielectric constant (εr), a high quality factor (Q·f) and a near-zero temperature coefficient of the resonant frequency (τfare required Nowadays a great number of new materials with excellent microwave dielectric properties have been widely investigated, such as the ReNbO4 system,5-7 Mo-based microwave dielectric ceramics,8-10 and the rock salt systems or others.11-13 For example, the LaNbO4-0.5MgO ceramics sintered at 1425 °C possessed excellent performance: εr=19.8, Q·f=94,440 GHz, τf=6.1 ppm/°C.6 The La2(Zr1-xTix)3(MoO4)9 ceramics achieved the best dielectric properties with εr= 10.33, Q·f= 80,658 GHz and τf= -16.80 ppm/℃.10

It was found in our previous work that the Li2ZrO3-MgO system sintered at 1500 °C exhibited excellent dielectric properties of εr=12.65, Q·f=165,924 GHz and τf=-34.66 ppm/°C.11 However, the high sintering temperature restricted its possible applications. In order to lower the sintering temperature, it is one of efficient methods to add or substitute sintering aids, such as LiF, CuO, H3BO3, Bi2O3 and glass in the past report.14-24 For example, when LiF content increased from 0 to 5 wt%, the optimum sintering temperature of CaMgSi2O6 ceramics reduced from 1250 °C to 900 °C.18 Zhou et al. reported that Bi2(Li0.5Ta1.5)O7 ceramics were densified at 1025 °C, the sintering temperature was lowered to 920 °C by the addition of 2 mol% excess Bi2O3.21 The H3BO3-doping in (1−x)LiAl0.98(Zn0.5Si0.5)0.02O2 + xCaTiO3 (0.05 ≤ x ≤ 0.20) ceramics was used to decrease the sintering temperature from 1150 °C to 900 °C.23 Among them, LiF is one of inexpensive as well as the most effective sintering additives to reduce the sintering temperature of microwave dielectric materials. Hence, a conventional solid-state reaction method was used to prepare Li6Mg7Zr3O16 ceramics doped with different amounts of LiF. The microstructures, sintering characteristics as well as microwave dielectric properties of Li6Mg7Zr3O16-x (0 wt% LiF ≤ x ≤ 5 wt% LiF) were investigated scientifically.


2. Experimental procedure

Li6Mg7Zr3O16 compositions was prepared using reagent-grade powders of MgO (99.99%, Aladdin), Li2CO3 (99.99%, Aladdin) and ZrO2 (99.99%, Aladdin). The raw materials were mixed by ball-milling for 24 h with zirconia balls and alcohol. The resulting slurry was dried after milling. After drying, powders were calcined at 1100 °C for 2 h thereafter. After calcination, powders were mixed together with 0-5 wt% LiF additives and then re-milled for 24 h. Powders were ground with 8 wt% PVA, and pressed into cylinders in a steel die thereafter. Finally, the matrix was sintered at 1300-1550 °C for 4 h in air, and Li6Mg7Zr3O16-x (1 wt% LiF ≤ x ≤ 5 wt% LiF) ceramics were sintered at 800-1150 °C.

The X-ray diffractometer (Brucker D8) was used to analyse crystal structures of Li6Mg7Zr3O16-x (0 wt% LiF ≤ x ≤ 5 wt% LiF). IR reflectivity spectrum was obtained via a FTIR spectrometer (Bruker IFS 66v). Microstructures of sintered samples were observed by the SEM (FeSEM Quanta 250, FEI Co., USA). The εr values was measured using Hakki-Coleman dielectric resonator method in TE011 resonant mode by A network analyzer (N5234A, Agilent Co., USA),25 the unloaded quality factor was measured using TE01d mode by the cavity method.26 τf values were calculated at 25-85 °C using the following equation:

where f0 and f1 represented the resonant frequency at 25 °C as well as 85 °C, respectively.


3. Results and discussion

Fig. 1 XRD patterns of Li6Mg7Zr3O16 ceramics sintered at 1300-1500 °C.

The X-ray diffraction patterns of Li6Mg7Zr3O16 samples were shown in Fig. 1.11 From the recorded XRD patterns, Li6Mg7Zr3O16 with a cubic structure (JCPDS care no. 45-0946) was identified. Moreover, no other phases could be detected, indicating that the pure cubic Li6Mg7Zr3O16 was formed in the temperature range 1300-1500 °C. The illustrations of Li6Mg7Zr3O16 crystal were exhibited in Fig. 2. It was noted that the oxygen octahedral sites were occupied by the Li, Mg and Zr atoms. In the structure, the cations (Li, Mg and Zr) occupied the 4a Wyckoff position, and O anions occupied the 4b Wyckoff position. According to complex chemical bond theory, the result of the decomposition of Li6Mg7Zr3O16 was exhibited in Eq. (2).27-29 Fig. 2 displayed the charge distribution of ions and the coordination number in Li6Mg7Zr3O16 samples. The coordination numbers of Li, Mg, and Ti were 6. The effective valences of cations were ZLi=1, ZMg=2 and ZTi=4, while effective valence of the anion (O) was closely related to the charge balance in the specific chemical bond, where ZO=-1 in Li-O bond, ZO=-2 in Mg-O bond as well as ZO=-4 in Ti-O bond.

                                      (2)

Fig. 2 The schematic crystal structure of Li6Mg7Zr3O16 ceramic and the coordination number and charge distribution of ions in Li6Mg7Zr3O16 ceramic.

Based on our previous work11, the shrinkage ratios and the apparent densities of the Li6Mg7Zr3O16 samples could reach the saturated values at 1500 °C during the temperature increasing from 1300 to 1550°C. Correspondingly according to the curves of dielectric constants and quality factors for Li6Mg7Zr3O16 samples from  1300 to 1550 °C, it was noted that dielectric constant increased from 13.37 to 14.28 with increasing temperature, which was caused by the elimination of pores. Then, the dielectric constant reached a saturated value in the temperature range 1500-1550 °C. The best Q·f value of Li6Mg7Zr3O16 samples increased to 81,284 GHz (at 8.94 GHz) as the sintering temperature increased to 1500 °C. Moreover, the τf value of Li6Mg7Zr3O16 sample sintered at the optimum sintering temperature (1500 °C) was -21.56 ppm/°C.

            Fig. 3 Measured (black line) and fitted (red line) IR reflectivity spectrum of Li6Mg7Zr3O16 ceramic sintered at 1500 °C.

Fig. 3 presented the IR reflectivity spectrum of Li6Mg7Zr3O16 sintered at 1500 °C. Eq. (3) is used to calculate ε*(ω) (the complex dielectric permittivity) based on the model of classical harmonic oscillator, and R (the complex reflectivity) can be obtained as Eq. (4)30,31

where ωoj, ωpj and ε are the transverse frequency, intensity and dielectric constant, respectively; n as well as γj are the number of transverse phonon modes and damping factor, respectively. In addition, the following formulas were used to calculate the tan δ (dielectric loss tangent):

Fig. 4 Real and imaginary parts of complex permittivity for Li6Mg7Zr3O16 ceramic sintered at 1500 °C (points are measured values at microwave region). 

 

Table 1 Phonon parameters obtained from the fitting of the infrared spectra of Li6Mg7Zr3O16 ceramic sintered at 1500 °C

mode

ωoj

ωpj

γj

Δεj

ε

ε*(ω)

1

277.41

855.9

86.452

9.52

4.06

22.88

2

308.73

517.36

44.705

2.81

 

 

3

378.39

855.4

118.34

5.11

 

 

4

513.88

440.03

69.187

0.733

 

 

5

1206.1

973.42

1173.3

0.651

 

 

 

Fig. 4 showed the complex permittivity values and fitted IR reflectivity spectra. As shown in Table 1, there were five internal modes. The extra-polated dielectric loss and permittivity of Li6Mg7Zr3O16 were 0.44×10-4 and 22.88, respectively. These calculated results were comparable with the measured ones. Hence, the microwave dielectric properties of Li6Mg7Zr3O16 were mainly related to the absorptions of phonon oscillation.

Fig. 5 Apparent densities of the Li6Mg7Zr3O16-x wt% LiF (x=1-5) ceramics sintered at 800-1150 °C.

The apparent densities of Li6Mg7Zr3O16-x (1 wt% LiF ≤ x ≤ 5 wt% LiF) ceramics were plotted in Fig. 5. Apparent densities of Li6Mg7Zr3O16-x (1 wt% LiF ≤ x ≤ 5 wt% LiF) ceramics initially increased to the maximum values at their optimum temperatures and then reached saturation with further increasing the temperatures. The apparent densities of Li6Mg7Zr3O16-x (3 wt% LiF ≤ x ≤ 5 wt% LiF) ceramics were higher than that of Li6Mg7Zr3O16-x (1 wt% LiF ≤ x ≤ 2 wt% LiF). Especially, the apparent densities of Li6Mg7Zr3O16-x wt% LiF (x=5) increased to 3.54 g/cm3 approximately at 900 °C, which was similar to that (3.66 g/cm3) of Li6Mg7Zr3O16 sintered at 1500 °C. In addition, the apparent density increased with increasing x value. Therefore, LiF additive was an effective sintering aid to lower sintering temperature of Li6Mg7Zr3O16 system.

Fig. 6 XRD patterns of Li6Mg7Zr3O16-x wt% LiF (x=1-5) ceramics sintered at 1100 °C in air.

Fig. 6 showed XRD patterns of Li6Mg7Zr3O16-x (1 wt% LiF ≤ x ≤ 5 wt% LiF) samples sintered at 1100 °C. For Li6Mg7Zr3O16-x (1 wt% LiF ≤ x ≤ 2 wt% LiF) compositions, main cubic phase was observed. The second phases were ZrO2 and Li2MgZrO4 for x=1 and x=2, respectively. As the LiF content increased from 3 to 5 wt%, a single phase was formed in the entire composition range, suggesting that Li6Mg7Zr3O16-x (x=3-5 wt% LiF) was a complete solid solution with a cubic structure. Typical SEM micrographs of Li6Mg7Zr3O16-x (1 wt% LiF ≤ x ≤ 5 wt% LiF) samples were demonstrated in Fig. 7(a-e). The porous structures were revealed in the surface of samples. It was observed that relatively dense microstructures were obtained for Li6Mg7Zr3O16 sample doped with 5 wt% LiF.

Fig. 7 SEM micrographs of Li6Mg7Zr3O16-x wt% LiF (x=1-5) ceramics sintered at 1100 °C (a-e corresponding to x=1, 2, 3, 4, 5).

Fig. 8 Dielectric constants of Li6Mg7Zr3O16-x wt% LiF(x=1-5)ceramics sintered at 800-1150 °C.

Fig. 8 illustrated the variation in the dielectric constants for Li6Mg7Zr3O16-x (1 wt% LiF ≤ x ≤ 5 wt% LiF). Generally, εr value is closely related to density and secondary phase. For the specimens with x =1- 2, dielectric constants had relatively lower values compared with that of Li6Mg7Zr3O16-x (3 wt% LiF ≤ x ≤ 5 wt% LiF) ceramics, which might be caused by the second phases and the apparent densities. For x=3-5, the XRD patterns in Fig. 6 showed a pure phase. Therefore, εr values of Li6Mg7Zr3O16-x (3 wt% LiF ≤ x ≤ 5 wt% LiF) were mainly dependent on densities. With temperature increasing from 900 to 1100 °C, it was seen that the εr values of Li6Mg7Zr3O16-x (3 wt% LiF ≤ x ≤ 5 wt% LiF) remained stable at a given LiF content, indicating that εr value was not significantly influenced by the temperature for the sample with high densification.32

Fig. 9 Quality factors of Li6Mg7Zr3O16-x wt% LiF (x=1-5) ceramics sintered at 850-1150 °C.

Quality factors of Li6Mg7Zr3O16-x (1 wt% LiF ≤ x ≤ 5 wt% LiF) samples were exhibited in Fig. 9. Quality factors are mainly influenced by the intrinsic losses (lattice vibration mode) and extrinsic losses (densification of the samples, second phases as well as grain morphology).33,34 Factors influencing the quality factors of Li6Mg7Zr3O16-x (1 wt% LiF ≤ x ≤ 5 wt% LiF) ceramics were the extrinsic ones mainly contributing to the densification. Quality factors of Li6Mg7Zr3O16-x (3 wt% LiF ≤ x ≤ 5 wt% LiF) ceramics were higher than that of Li6Mg7Zr3O16-x (1 wt% LiF ≤ x ≤ 2 wt% LiF) samples, which might be related to the densities and the second phases of the ceramics. For instance, quality factor of Li6Mg7Zr3O16-x wt% LiF (x=1) increased from 8,200 GHz (at 10.49 GHz) to 75,500 GHz (at 10.67 GHz) with increasing temperature, while the maximum quality factors of Li6Mg7Zr3O16-5 wt% LiF ceramics reached to 132,600 GHz (at 9.26 GHz) at 1100 °C, which was higher than that (81,277 GHz) of Li6Mg7Zr3O16. Therefore, it could be found that the appropriate LiF contents might improve the sintering characteristis without any deterioration on quality factors of Li6Mg7Zr3O16 ceramics.

Fig. 10 Microwave dielectric properties of the Li6Mg7Zr3O16 ceramics doped with 1-5 wt% LiF sintered at 1100 °C.

Microwave dielectric properties of Li6Mg7Zr3O16-x (1 wt% LiF ≤ x ≤ 5 wt% LiF) sintered at 1100 °C were exhibited in Fig. 10. As LiF content increased from 1 to 5 wt%, the εr value and the quality factor showed an upward tendency due to the increase of density. It is well known that the τf values are governed by the composition, the additives and the second phase of the materials.35 The τf value initially decreased from -19.5 ppm/°C to -27.2 ppm/°C with increasing x value, and then increased to -18.89 ppm/°C, which might be affected by additives in this work. Typically, Li6Mg7Zr3O16-x wt% LiF (x=5) ceramic possessed a single phase with good properties of τf=-18.89 ppm/°C, Q·f=132,600 GHz (at 9.26 GHz) and εr=13.67.


4. Conclusion

Li6Mg7Zr3O16-x (0 wt% LiF ≤ x ≤ 5 wt% LiF) samples were synthesized by the solid-state method. Appropriate amount of LiF additive improved the sinterability of the Li6Mg7Zr3O16 system. The sintering temperature of the ceramic was reduced with increasing LiF content, which was caused by the enhancement of the apparent density at low temperature by liquid phase sintering. The dielectric constants and quality factors increased gradually when temperatures and LiF additives increased. Compared with Li6Mg7Zr3O16 ceramics sintered at 1500°C, 5% LiF doped-Li6Mg7Zr3O16 samples could be sintered well at 1100°C with excellent properties of εr=13.67, Q·f=132,600 GHz (at 9.26 GHz) as well as τf=-18.89 ppm/°C .


Acknowledgments

This work was supported by the National Natural Science Foundation of China (No. 51472108). The authors are thankful to the help of Professor Zhen Xing Yue and postdoctoral Jie Zhang on the measurement of microwave properties in Tsinghua University. The authors are also thankful to the administrators in IR beamline workstation of National Synchrotron Radiation Laboratory (NSRL) for the help in IR measurement.


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