Rare Earth Ion-Doped Y_2.95R_0.05MgAl₃SiO₁₂ (R = Yb, Y, Dy, Eu, Sm) Garnet-Type Microwave Ceramics for 5G Application

Abstract

In this work, Y_2.95R_0.05MgAl₃SiO₁₂ (R = Yb, Y, Dy, Eu, Sm) microwave single-phase dielectric ceramics were successfully prepared via a conventional ceramic sintering technology by doping a series of rare earth elements (Yb, Y, Dy, Eu, Sm) with different ionic radii for the first time. The effects of A-sites occupied by rare earth elements on the microwave dielectric properties of Y_2.95R_0.05MgAl₃SiO₁₂ were studied using crystal structure refinement, a scanning electron microscope (SEM), bond valence theory, P-V-L theory, and infrared reflection spectroscopy. It was found that the ionicity of the Y-O bond, the lattice energy, the bond energy, and the bond valance of the Al_(Tet)-O bond had important effects on the microwave dielectric properties. Particularly, the optimum microwave dielectric properties, ε_r = 9.68, Q × f = 68,866 GHz, and τ_f = −35.8 ppm/°C, were obtained for Y_2.95Dy_0.05MgAl₃SiO₁₂ when sintered at 1575 °C for 6 h, displaying its potential for 5G communication.

1. Introduction

With the rapid development of communication frequency bands to millimeter-wave bands, the microwave dielectric ceramic materials used in communication equipment are required to include the following dielectric properties:

(1) low ε_r to reduce delay in the signal transmission process; (2) ultra-high Q × f values to reduce the transmission loss; (3) near-zero temperature coefficients (τ_f), which can improve the device stability in different environments when applied in resonators, antennas, filters, 5G base stations, etc. [1,2,3,4,5,6].

In low dielectric constant material systems, the Y₃Al₅O₁₂ garnet has attracted extensive research due to its low ε_r and high Q × f value in 5G communication systems [7]. Figure 1 shows the Q × f values of the various types of garnet-type microwave dielectric ceramics, including the vanadate garnet, aluminate garnet, etc. [8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24]. It is clear that the Q × f value of the aluminate garnet is much higher than that of others. The aluminate garnet has the formula of Y₃Al₅O₁₂ (YAG), in which three Y³⁺ ions occupy the dodecahedral A-site, two Al_(Oct)³⁺ ions occupy the octahedral B-site, and three Al_(Tet)³⁺ ions occupy the tetrahedral C-site. The Q × f value of Y₃Al₅O₁₂ microwave ceramics was initially reported to be as high as 440,000 GHz [25]. Later, Jin et al. [12] reported the excellent microwave dielectric properties of ε_r = 10.8, Q × f = 213,400 GHz, and τ_f = −30 ppm/°C for Y₃Al₅O₁₂ ceramics pressed under 200MPa by a cold isostatic pressing technology and sintered at 1750 °C for 5 h in a vacuum environment. Zhou et al. [15] synthesized Y₃Al_4.97Mg_0.03O_11.985 microwave ceramics by replacing Al_(Oct)³⁺ with Mg²⁺ and sintering at 1700 °C for 12 h, which showed the following excellent microwave dielectric properties: ε_r = 10.9, Q × f = 218,168 GHz, and τ_f = −30 ppm/°C. Then, non-stoichiometric YAG ceramics (Y_3.03Al₅O₁₂) were further synthesized at 1750 °C for 12 h, showing the following good microwave dielectric properties: ε_r = 11.2, Q × f = 236,936 GHz, and τ_f = −35.9 ppm/°C [16]. However, the sintering temperature of YAG ceramics was too high (>1700 °C), which does not conform to the concept of low carbon and environmental protection. In order to solve the problems of high-sintering temperatures and large τ_f values, much research has been carried out. Zhang et al. [14] reported that the sintering temperature of YAG ceramics was reduced from 1700 °C to 1360 °C by using LiF as an additive, producing properties of Q × f = 89,810 GHz, ε_r = 10.63, and τ_f = −51.4 ppm/°C. Peng et al. [26] reported a near-zero τ_f value (+7 ppm/°C) for Ca²⁺ and Ti⁴⁺ co-doped Ca_1.5Y_1.5Al_3.5Ti_1.5O₁₂ ceramics, as well as ε_r = 32.6 and Q × f = 45,200 GHz.

Previous reports have shown that a MgO-SiO₂ liquid phase was formed in Y₃Al₅O₁₂ garnet ceramics with MgO and SiO₂ as sintering aids, which improved the densification rate of the ceramics [27]. Compared with YAG ceramics, Y₃MgAl₃SiO₁₂ ceramics were formed by doping Mg²⁺ at the B-site octahedrons and Si⁴⁺ at the C-site tetrahedrons of YAG, which reduced the sintering temperature from 1670 °C to 1550 °C and exhibited the good microwave dielectric performances of ε_r = 10.1, Q × f = 57,340 GHz, and τ_f = −32 ppm/°C [13,28,29]. The τ_f of Y₃MgAl₃SiO₁₂ has been further tuned to a near-zero value (+5.2 ppm/°C) by forming composites with 0.2TiO₂ [30]. However, the modification of A-site dodecahedrons for garnet ceramics has been the subject of very little research. Herein, we have designed a scheme of A-site ionic substitution for the Y element in Y₃MgAl₃SiO₁₂ ceramics using a series of rare earth elements with different ionic radii (Yb, Y, Dy, Eu, Sm). The microwave dielectric properties of Y_2.95R_0.05MgAl₃SiO₁₂ (R = Yb, Y, Dy, Eu, Sm) ceramics were well discussed using crystal structure refinement, bond valence theory, P–V–L theory, and infrared reflectance spectrum.

2. Experimental Process

Y_2.95R_0.05MgAl₃SiO₁₂ (R = Yb, Y, Dy, Eu, Sm) ceramics were prepared using raw materials of Yb₂O₃ (Shanghai Aladdin Reagent Co., Ltd., Shanghai, China, 99.99%), Y₂O₃ (Shanghai Aladdin Reagent Co., Ltd., 99.99%), Dy₂O₃ (Shanghai Aladdin Reagent Co., Ltd., 99.99%), Eu₂O₃ (Shanghai Aladdin Reagent Co., Ltd., 99.99%), Sm₂O₃ (Shanghai Aladdin Reagent Co., Ltd., 99.99%), MgO (Shanghai Aladdin Reagent Co., Ltd., 99.99%), Al₂O₃ (Shanghai Aladdin Reagent Co., Ltd., 99.99%), and SiO₂ (Shanghai Aladdin Reagent Co., Ltd., 99.99%). Raw materials were weighed according to the stoichiometric ratio and planetarily ball-milled for 12 h in solvent ethanol. The speed for milling was 240 r/min. The mixed slurries were dried at 80 °C, and then the dried powders were calcined at 1400 °C for 4 h. The calcined powder was re-milled and mixed uniformly with 5 wt% organic binders (polyvinyl alcohol). The granulated powder was sieved using a 60-mesh sieve and pressed into cylindrical green pellets with a diameter of 12 mm and a height of ~7 mm. The green pellets were first fired at 800 °C for 4 h to remove the binder and then sintered at 1500 °C–1650 °C for 6 h.

The crystal structure was identified by X-ray powder diffraction (XRD) (Shimadzu, Kyoto, Japan) using Cu Kα radiation at the range of 2θ from 10° to 80°, with a step size of 0.02°. The GSAS software was used to analyze the crystal structure parameters of XRD data [31,32]. The microstructure of the sintered samples was observed by a field emission scanning electron microscope (SEM, Sigma 300, ZEISS, Oberkochen, Germany). The Archimedes method was used to determine the bulk density. The infrared reflectance spectra were recorded using the Bruker IFS 66v beam line of the Hefei National Synchrotron Radiation Laboratory. Microwave dielectric properties were measured in TE_01δ mode using the resonant cavity method. The Keysight (N5234B) vector network analyzer was used for evaluating the Q × f values and ε_r. The τ_f value was calculated by the following formula [33]:

$${\tau }_f=\frac{f_2-f_1}{f_1\times \left(T_2-T_1\right)}\times {10}^6\left(\mathrm{ppm}/℃\right)$$

(1)

where f₁ and f₂ were the resonant frequency at 25 and 85 °C, respectively.

3. Results and Discussion

The XRD patterns of Y_2.95R_0.05MgAl₃SiO₁₂ (R = Yb, Y, Dy, Eu, Sm) ceramics are displayed in Figure 2. The diffraction peaks of all samples match well with the YAG structure (PDF No. 88-2047), indicating the formation of a garnet solid solution. It can be clearly seen that the diffraction peaks move to the lower 2θ angle with the increase in R ionic radius (Yb³⁺—0.985 Å, Y³⁺—1.019 Å, Dy³⁺—1.027 Å, Eu³⁺—1.066 Å, Sm³⁺—1.079 Å) from magnified spectra in Figure 2b. The XRD data of the Y_2.95R_0.05MgAl₃SiO₁₂ ceramics, which are shown in Figure S2a–e, are analyzed using the Rietveld method.

Table 1. The crystallographic data obtained by Rietveld refinement for Y_2.95R_0.05MgAl₃SiO₁₂ ceramics.

R	Yb	Y	Dy	Eu	Sm
Crystal system	cubic
Space group	Ia-3d
Z	8
a = b = c(Å)	12.0482	12.0499	12.0529	12.0589	12.0668
α = β = γ(°)	90
Vcell(Å3)	1749.121	1749.607	1750.103	1750.623	1751.009
Calc.density(g/cm3)	4.602	4.357	4.538	4.527	4.417
Rwp(%)	9.17	10.5	9.8	10.1	9.8
Rp(%) χ2 Y/R-O (Å) (Al(Oct)/Mg)-O (Å) (Al(Tet)/Si)-O (Å)	6.34 4.36 2.2932 2.4466 2.0038 1.7355	7.38 4.35 2.3224 2.4770 1.9881 1.7257	8.37 2.65 2.3002 2.4782 2.0062 1.7352	8.87 3.06 2.3106 2.4796 1.9894 1.7387	6.5 2.64 2.3329 2.4865 1.9649 1.7528

lists the detailed refined parameters. Low Rietveld discrepancy factors (R_wp~9%, R_p~7%, χ²~4) are obtained, suggesting that the refinement results are reliable. The unit cell volume of Y_2.95R_0.05MgAl₃SiO₁₂ ceramics increases slightly with increasing R ionic radius, which is consistent with the diffraction peak’s shift toward the lower 2θ direction. The schematic crystal structure of Y_2.95R_0.05MgAl₃SiO₁₂ ceramics is given in Figure S1f.

The SEM images of the Y_2.95R_0.05MgAl₃SiO₁₂ (R = Yb, Y, Dy, Eu, Sm) ceramics sintered at the optimal sintering temperature (Yb—1600 °C, Y—1600 °C, Dy—1575 °C, Eu—1600 °C, Sm—1600 °C) are shown in Figure 3a–e. All sintered ceramics are dense except for the Sm-doped sample, which has obvious voids. The grain size distribution of each sample is shown in Figure S2 (Supplementary Materials), and the average grain size is plotted in Figure 3f. Among all samples in this study, Y_2.95Dy_0.05MgAl₃SiO₁₂ ceramic has the largest average grain size, indicating that Dy³⁺-doping could be conducive to the densification and growth of ceramics.

Figure 4 exhibits the microwave dielectric properties of Y_2.95R_0.05MgAl₃SiO₁₂ (R = Yb, Y, Dy, Eu, Sm) ceramics sintered at the optimal temperature. The ε_r values show a gradually increasing trend, except for Dy, which has a lower ε_r value of 9.68. The Q × f values are in the range of 47,000 GHz ~ 70,000 GHz, which is consistent with the trend of relative density (ρ_r). The τ_f value is between −38.7 ppm/°C and −28.6 ppm/°C. It is widely known that the microwave dielectric properties are dependent on both extrinsic (second phase, density, grain size, etc.) and intrinsic (lattice vibration) factors [34]. The relative densities of Y_2.95R_0.05MgAl₃SiO₁₂ ceramics sintered at the optimal sintering temperature are high (ρ_r > 94%), and no secondary phases could be detected. Therefore, the intrinsic factors, such as the crystal structure and chemical bonds, played a decisive role in the dielectric properties. Herein, the relationship between the microwave dielectric properties and internal factors of the Y_2.95R_0.05MgAl₃SiO₁₂ ceramics is discussed using the P–V–L theory. The detailed calculation methods are included in the Supplementary Materials [35,36,37,38].

In general, the measured permittivity (ε_r) is related to the bond ionicity (f_i). The calculated results of f_i are listed in Table S1 (Supplementary Materials). In addition, the theoretical permittivity (ε_theo) of Y_2.95R_0.05MgAl₃SiO₁₂ ceramics can be calculated using the Clausius-Mosotti Equations (2) and (3) [39,40]:

$${\mathsf{\epsilon }}_{theo}=\frac{3}{1-b\mathsf{\alpha }/V_m}-2$$

(2)

$$V_m=\frac{V_{cell}}{Z}$$

(3)

Moreover, the corrected dielectric constant (ε_c) by porosity (P) can be calculated by Equations (5) and (6) [41]:

$$\mathrm{P}=1-{\mathsf{\rho }}_{\mathrm{r}}$$

(4)

$${\mathsf{\epsilon }}_{\mathrm{c}}={\mathsf{\epsilon }}_{\mathrm{r}}\left(1+1.5\mathrm{P}\right)$$

(5)

As shown in Figure 5a, the ε_r is consistent with the changing trend of ε_theo, ε_c, and the average bond ionicity (Δf_i). The average ionicity properties of the Y-O, Al_(Oct)-O, and Al_(Tet)-O bonds of Y_2.95R_0.05MgAl₃SiO₁₂ (R = Yb, Y, Dy, Eu, Sm) are given in Figure 5b. The maximum value of f_i is 94.91% for the Y-O bond, indicating that the Y-O bond plays a dominated role in affecting the ε_r value of Y_2.95R_0.05MgAl₃SiO₁₂ ceramics.

The lattice vibration of microwave dielectric ceramics has a great influence on dielectric loss. The lattice energy of the chemical bonds in microwave dielectric ceramics can be used to effectively evaluate the lattice vibration of ceramics [42]. Therefore, we can use the average lattice energy (U) value to predict the Q × f values, and the calculation results of the average lattice energy (U) value are listed in Table S2. The U, grain size, and Q × f values of the Y_2.95R_0.05MgAl₃SiO₁₂ ceramics are shown in Figure 6a. It can be seen that the U is consistent with the trend of Q × f values of Y_2.95R_0.05MgAl₃SiO₁₂ ceramics, suggesting that the U is an important factor affecting the Q × f values of Y_2.95R_0.05MgAl₃SiO₁₂ ceramics. Figure 6b shows U of the Y-O bonds, Al_(Oct)-O bonds, and Al_(Tet)-O bond in Y_2.95R_0.05MgAl₃SiO₁₂ ceramics (Al_(Tet)-O (33,533 kJ/mol)> Y-O(22,143 kJ/mol) > Al_(Oct)-O (21,989 kJ/mol)); it indicates that the Al_(Tet)-O bond plays a dominated role in determining the Q × f value. In addition, a larger average grain size shows fewer grain boundaries, which means higher Q × f values [43].

The τ_f is related to the bond valence (V_ij) and the bond energy (E). The E represents the strength of chemical bonds, which is generally evaluated by the amount of energy required to break the chemical bonds. The smaller the V_ij, the smaller the E required to recover the oxygen polyhedral deformation, leading to a decrease in the τ_f value. The V_ij value of Y_2.95R_0.05MgAl₃SiO₁₂ ceramics is calculated by Equations (7) and (8) [44,45]:

$$v_{ij}=exp\left\{\frac{R_{ij}-d_{ij}}{B}\right\}$$

(6)

$${\mathrm{V}}_{ij}={\displaystyle \sum }_j^i{v}_{ij}$$

(7)

where R_ij is the bond valence parameter, B is a constant (0.37 Å), and d_ij is the bond length. The calculated results for E and V_ij are listed in Tables S3 and S4 (Supplementary Materials). The E, the V_ij of Al_(Tet)-O, and the τ_f value are shown in Figure 7a. It can be observed that the τ_f value of Y_2.95R_0.05MgAl₃SiO₁₂ ceramics fluctuates from −28.6 to −38.7 ppm/°C, which is consistent with the changing trend of average E and V_ij. Figure 6b shows the average E of Y-O, Al_(Oct)-O, and Al_(Tet)-O bonds (Al_(Tet)-O (307.28 kJ/mol) > Al_(Oct)-O (224.10 kJ/mol) > Y-O (218.47 kJ/mol)), which indicates that the Al_(Tet)-O bond plays a major role in the temperature stability of Y_2.95R_0.05MgAl₃SiO₁₂ ceramics.

In order to further analyze the inherent microwave dielectric properties of Y_2.95R_0.05MgAl₃SiO₁₂ ceramics, the infrared reflectance spectrum was analyzed based on the classical harmonic oscillator model:

$$\mathrm{R}\left(\mathsf{\omega }\right)={\left|\frac{\sqrt{{\mathsf{\epsilon }}^{*}\left(\mathsf{\omega }\right)}-1}{\sqrt{{\mathsf{\epsilon }}^{*}\left(\mathsf{\omega }\right)}+1}\right|}^2$$

(8)

$${\mathsf{\epsilon }}^{*}\left(\mathsf{\omega }\right)= \mathsf{\epsilon }\prime \left(\mathsf{\omega }\right)-\mathrm{i}\mathsf{\epsilon }″\left(\mathsf{\omega }\right)={\mathsf{\epsilon }}_{\infty }+{{\displaystyle \sum }}_{\mathrm{j}=1}^{\mathrm{n}}\frac{{\mathrm{S}}_{\mathrm{j}}}{{\mathsf{\omega }}_{\mathrm{j}}^2-{\mathsf{\omega }}^2+{\mathrm{i}\mathsf{\omega }\mathsf{\gamma }}_{\mathrm{j}}}$$

(9)

The relevant parameters in the formula were described in detail in the previous literature [46,47]. The infrared reflectance spectrum can be well-fitted with ten modes in Figure 8a. Table S5 (Supplementary Materials) lists the relevant phonon parameters. For Y_2.95Dy_0.05MgAl₃SiO₁₂ ceramics, the theoretical ε_r (~8.55) at 10.86 GHz in Figure 8b,c, is less than the measured value (~9.68). The calculated Q × f value is 89,752 GHz (f = 10.86 GHz, Q = 1/tanδ, and tanδ = 1.21 × 10⁻⁴), which is greater than the measured value of 68,868 GHz. Differences between the measured and fitted values are because of the extrinsic loss affected by all kinds of defects [48].

4. Conclusions

In this paper, the single-phase ceramics Y_2.95R_0.05MgAl₃SiO₁₂ (R = Yb, Y, Dy, Eu, Sm) were successfully prepared using a conventional ceramic sintering technology. The relationship between the crystal structure, microstructure, and microwave dielectric properties of Y_2.95R_0.05MgAl₃SiO₁₂ (R = Yb, Y, Dy, Eu, Sm) ceramics was analyzed by crystal structure refinement, SEM, bond valence theory, P–V–L theory, and infrared reflectance spectrum. The ε_r of Y_2.95R_0.05MgAl₃SiO₁₂ ceramics was mainly affected by the f_i of the Y-O bond. The τ_f value was mainly affected by the average E and bond valence of Al_(Tet)-O. In addition, infrared reflectance spectrum demonstrated that the calculated Q × f value was greater than the measured value, indicating the effect of extrinsic factors on the Q × f value. In particular, the microwave dielectric properties were obtained for Y_2.95Dy_0.05MgAl₃SiO₁₂, sintered at 1575 °C for 6 h, with ε_r = 9.68, Q × f = 68,866 GHz, and τ_f = −35.8 ppm/°C. The results show that Y_2.95Dy_0.05MgAl₃SiO₁₂ garnet ceramics have potential in 5G communication frequency bands, such as dielectric substrates, microstrip patch antenna, etc.