Microwave Dielectric Properties of Li₃TiO₃F Oxyfluorides Ceramics

Abstract

Using a solid-state reaction strategy, nominal Li₃TiO₃F oxyfluorides ceramics were fabricated, and its sintering behavior, microstructure, phase assemblages, as well as microwave dielectric performances were all investigated. The main phase of Li₃TiO₃F with cubic structures accompanied with small amounts of the LiF or Li₂TiO₃ secondary phase was identified by XRD analysis. SEM analysis showed that a uniform and dense microstructure was obtained for 750 °C-sintered samples. The dielectric constant (ε_r) and quality factor (Q × f) were found to be strongly correlated with porosity and grain size distribution, whereas the temperature coefficient of resonance frequency (τ_f) was mainly dominated by the phase assemblages. In particular, the 750 °C-sintered Li₃TiO₃F samples exhibited good microwave dielectric performances: ε_r = 18, Q × f = 57,300 GHz (under 9.2 GHz), τ_f = −43.0 ppm/°C.

1. Introduction

Microwave dielectrics ceramics (MDCs) have become extraordinarily alive with the advent of the fifth-generation (5G) mobile network and the Internet of Things (IoT), which have been widely utilized in miscellaneous microwave devices [1,2]. Among the MDCs, low temperature co-fired ceramics (LTCCs) are able to simultaneously hold low firing temperatures (≤960 °C), moderate permittivity (ε_r), and possess a high quality factor (Q × f), as well as approaching a zero temperature coefficient of resonant frequency (τ_f), which is profitable for the assembling of electronic devices and environmental protection [3,4]. Thus, it is necessary to explore these MDCs, especially the LTCCs, to better meet the current demands for the IoT and 5G era [5,6,7,8,9].

Recently, a series of oxyfluorides have been prepared either by solid solution or anion substitution, which exhibited inherently low sintering temperatures and excellent microwave dielectric performances concurrently [10,11,12]. For example, in 2019 the structure and microwave dielectric performances of novel Ti-containg oxyfluorides, such as Li₇Ti₃O₉F and Li₅Ti₂O₆F were first reported by Fang et al. [10,11]. Subsequently, several binary, ternary, and multicomponent Nb-containing oxyfluorides with promising microwave dielectric performances, such as Li_5.5Nb_1.5O₆F, Li₄Mg₂NbO₆F, and Li₇(Nb_1−xTi_x)2O_8−xF, Li₆MgTiNb_1−xV_xO₈F, have been reported by Liu et al. [13,14,15]. In 2023, Zhang et al. reported a new oxyfluoride dielectric ceramic system of Li_2+xZrO₃F_x, among which the Li₃ZrO₃F ceramics simultaneously exhibited a near zero τ_ƒ (1.2 ppm/°C), a high Q × f (65,100 GHz), and a low firing temperature (925 °C) [16]. The abovementioned research opened a scheme to develop novel LTCCs with superior dielectric performances [16]. Li-containing rock salt structured Li₂AO₃ (A = Ti, Sn, Zr) system ceramics have drawn a tremendous amount of attention due to their promising microwave dielectric performances (ε_r = 13~22, τ_f = 20~38 ppm/°C and Q × f = 38,000~120,000 GHz) [17,18]. However, the high heating temperature (≥1300 °C) and non-near zero τ_f have severely impeded the commercial application of Li₂AO₃ ceramics. Through a mixture of Li₂TiO₃ and LiF, along with subsequent sintering, the Li₃TiO₃F major phase has been synthesized by Szymanski and Bian et al. [19,20]. However, the Q × f (~30,000 GHz) of nominal Li₃TiO₃F ceramics is not high due to the appearance of the LiF second phase. Until now, there are no relative reports on pure phase Li₃TiO₃F ceramics along with their microwave dielectric properties. Thus, in this study, we aimed to fabricate a pure Li₃TiO₃F compound using a solid-state reaction route, and its sinterability, phase assemblages, microstructures and microwave dielectric performances were all further investigated.

2. Materials and Methods

Li₃TiO₃F oxyfluorides were synthesized following a solid-state reaction route. According to the stoichiometric formula of Li₃TiO₃F, the raw materials of Li₂CO₃ (98%, Guo-Yao Co. Ltd., Shanghai, China), LiF (98%, Guo-Yao Co. Ltd., Shanghai, China), and TiO₂ (99.9%, Guo-Yao Co., Ltd., Shanghai, China) were individually weighed, and then milled for 8 h using a planetary ball mill with a milling rate of 350 r/min. After drying, these powders were calcined under 600 °C for 4 h. The prebaked powders were reground after crushing, granulated with 6 wt.% solution of polyvinyl alcohol, and then pressed into cylindrical discs (12 mm-diameter and 6 mm-thick) at 100 MPa. Finally, these cylindrical discs were agglutinated under 500 °C for 2 h to remove the binder, and then fired at 700–800 °C for 5 h. In order to compensate for the volatilization of Li and F during sintering, the samples were muffled with sacrificial powders owing the same composition and in a covered crucible.

The crystalline phases were characterized by X-ray diffraction (XRD, Smartlab, Japan) with CuK_α radiation. XRD data for Rietveld refinement were collected in the range of 10−80°, with a step size of 0.02°, and a count time of 1 s. The lattice parameters, phase quantity, and theoretical density of the sample were refined and calculated via GSAS software and Equation (1) (shown below) [21,22]. In Equation (1), % and ρ_m represent the weight percentage and the theoretical density of the given phase, respectively. The bulk densities of the Li₃TiO₃F ceramics were assessed by Archimedes’ principle. The microstructures were observed with scanning electron microscopy (SEM, JSM-6610, Jeol, Tokyo, Japan). Using the Rohde & Schwarz network analyzer, the Hakki-Coleman dielectric resonator approach modified by Courtney [23,24] was utilized to measure the microwave dielectric performances of the Li₃TiO₃F ceramics. The τ_f was achieved by Equation (2):

$${\rho }_t=\frac{100}{\frac{\%\mathrm{phase}1}{{\rho }_m\mathrm{phase}1}+\frac{\%\mathrm{phase}2}{{\rho }_m\mathrm{phase}2}}$$

(1)

$${\tau }_f=\frac{f_{85}-f_{25}}{f_{25}\times (85-25)}\times {10}^6(\mathrm{ppm}/\mathrm{°C})$$

(2)

where f₂₅ and f₈₅ represent the resonant frequency at 25 °C and 85 °C, respectively.

3. Results and Analysis

The bulk density (ρ_b) and relative density (ρ_r) of the Li₃TiO₃F ceramics are illustrated in Figure 1. The theoretical density of sample was gauged from the refined XRD data, as shown in

Table 1. Refinement data of nominal Li₃TiO₃F ceramics sintered under different conditions.

S.T.	Phase	Phase Quantity	ρm	ρt	a = b = c	V	Rwp	Rp
(°C)		(%)	(g/cm−3)	(g/cm−3)	(Å)	(Å3)	(%)	(%)
725	Li3TiO3F	96.657	3.192	3.200	4.133	70.599	8.490	6.720
	Li2TiO3	3.343	3.430		8.277	566.980
750	Li3TiO3F	97.245	3.199	3.204	4.130	70.450	9.660	7.420
	Li2TiO3	2.755	3.407		8.295	570.830
775	Li3TiO3F	92.367	3.229	3.172	4.126	70.218	8.400	6.620
	LiF	7.633	2.622		4.035	65.707
800	Li3TiO3F	94.102	3.210	3.170	4.125	70.198	7.240	5.780
	LiF	5.898	2.637		4.028	65.337

. Following the rise in heating temperature from 700 °C to 750 °C, the ρ_b and ρ_r of Li₃TiO₃F sintered bodies were found to have enhanced from 2.870 g/cm³ to 3.150 g/cm³, and from 89.1% to 98.3%, respectively. The increase in ρ_r resulted from the removal of the porosity, whereas the abatement in ρ_r was due to the over-sintering [25]. From the view of density, the optimal sintering temperature of the Li₃TiO₃F ceramics was 750 °C. Compared to Li₂TiO₃ ceramics (1300 °C), the inherently low sintering temperature of the Li₃TiO₃F ceramics (750 °C) is benefited from the alleviation of the chemical potential caused by the co-occupied anion position of the F⁻ and O²⁻ ions, and a similar phenomenon was reported in our previous article [26].

Figure 2 displays the typical fresh fracture of nominal Li₃TiO₃F ceramics sintered at different temperatures. Several intergranular pores were observed for the 725 °C-sintered sample. The 750 °C-sintered sample exhibited a relatively uniform and compact microstructure with a mean grain size of around 210 nm, as shown in Figure 2b, corresponding to the achieved maximum relative density. However, when the sintering temperature surpassed 775 °C, poor grain uniformity and exaggerated grain growth appeared as shown in Figure 2c,d, which would therefore deteriorate the sample’s dielectric properties.

Figure 3 exhibits the XRD profiles of nominal Li₃TiO₃F specimens fired at 725~800 °C. For the samples sintered at 725 and 750 °C, their XRD patterns were identified as cubic structure Li₃TiO₃F (#04-002-4527) and Li₂TiO₃ (#01-075-1602) phases, and no diffraction peaks of LiF were observed. With increasing the temperature to 775 and 800 °C, except for the major phase Li₃TiO₃F, the diffraction peaks from the Li₂TiO₃ phase vanished, whereas the diffraction peaks from the LiF phase (#01-089-3610) appeared. In our experiment, pure phase Li₃TiO₃F was not obtained, and this was similar with the report published by Bian et al. [20], but somewhat different with the previous report by Szymanski et al. [19].

To further clarify the crystal structure information and phase assemblage, Rietveld refinements of the XRD data were conducted on the nominal Li₃TiO₃F ceramics via GSAS software using three phase models consisting of Li₃TiO₃F, LiF, and Li₂TiO₃. Figure 4 displays the comparison of the simulated and measured XRD profiles of Li₃TiO₃F specimens fired at 725~800 °C, and the resultant refined results are summarized in Table 1. As shown in Figure 4 and Table 1, small reliability factors below 10% were observed, suggesting that the refinement results obtained were creditable.

Figure 5 displays the ε_r values of nominal Li₃TiO₃F ceramics based on the heating temperatures employed. The ε_r was found to initially increase, reaching a maximum value under 750 °C, and then subsequently declined with further rising sintering temperatures. The change in the ε_r and ρ_r values with firing temperature illustrated an analogous variation tendency, suggesting that the ρ_r played a vital role in impacting the ε_r of current ceramics [27]. Moreover, for the samples sintered above 750 °C, the degradation of ε_r was also found to be connected with the disappearance of Li₂TiO₃ (ε_r = 22.0) and the occurrence of LiF (ε_r = 8.0) phases, respectively [17,28].

The variations of τ_f and Q × f in nominal Li₃TiO₃F ceramics are illustrated in Figure 6. The τ_f is dependent on phase constitution and crystal structure [8]. In this study, the τ_f exhibited a downward tendency from −42.0 ppm/°C to −48 ppm/°C as the sintering temperature increased from 725 °C, to 800 °C, respectively, which was attributed to the changed phase assemblages (Figure 3) since the LiF registered a negative τ_f (−117.0 ppm/°C), while the Li₂TiO₃ registered a positive τ_f (20.0 ppm/°C) [17,28]. In addition, as the sintering temperature rose from 725 °C, to 750 °C, the Q × f of Li₃TiO₃F ceramics gradually increased from 52,600 GHz to 57,300 GHz, respectively. Subsequently, the Q × f values showed a downward trend and ultimately obtained 54,400 GHz at 800 °C. In practical ceramics, the Q × f is typically dominated by the extrinsic loss rather than intrinsic losses corresponding to the electromagnetic field interaction with the phonons [29,30,31]. This extrinsic loss has been associated with microstructural characteristics (such as pores, grain morphology, grain boundaries, and secondary phases, etc.) [32]. The influence of the secondary phases of Li₃TiO₃ (Q × f = 63,500 GHz) and LiF (Q × f = 73,800 GHz) on the Q × f in present ceramics can be ignored due to their relative high Q × f values [17,28], as shown in Figure 3. Hence the porosity and grain size distribution were considered to determine the Q × f of present ceramics [32]. The enhancement of Q × f was associated with the synergistic effects of the enhancement of a uniform microstructure and reduction of porosity, whereas the reduction of Q × f was associated with the nonuniform and exaggerated grain growth (Figure 2). In addition, the Q × f value of present ceramics was found to be lower than those of Li₂TiO₃ and LiF, which may be connected with the disordered charge distribution in the crystal as reported in previous research [30,31].

Table 2. The sintering temperature (T_s), along with the microwave dielectric performances of several rock salt structured ceramics and present ceramics.

Compounds	εr	Q × f(GHz)	τf (ppm/°C)	TS (°C)	Ref.
Li7Ti3O9F	22.5	88 200	−24.0	950	[10]
Li5Ti2O6F	19.6	79 500	−30.0	880	[11]
Li3ZrO3F	15.8	65 100	1.0	925	[16]
Li2TiO3	22.0	63 500	20.0	1300	[17]
Li3TiO3F	18.6	30 000	−58.0	875	[20]
Li3TiO3F	18.0	57 300	−42.0	750	This study

summarizes the sintering temperature (T_s) along with the microwave dielectric performances of several rock salt structured ceramics and present ceramics. As shown in Table 2, although the microwave dielectric performances of the present ceramics are somewhat inferior to other counterparts, its remarkable advantages include the relatively low sintering temperature, which is conducive to energy conservation.

4. Conclusions

The relationships between the microstructure and microwave dielectric properties of Li₃TiO₃F ceramics were investigated in this study. XRD analysis showed that the major phase of Li₃TiO₃F accompanied with small amounts of Li₂TiO₃ and LiF phases were formed. Dense ceramics with a mean grain size of around 210 nm were obtained from Li₃TiO₃F sintered at 750 °C. As the sintering temperature increased, the ε_r and Q × f values first increased and then decreased, whereas its τ_f decreased slightly. Typically, the nominal Li₃TiO₃F ceramics fired at 750 °C displayed favorable microwave dielectric properties: ε_r =18.0, Q × f = 57,300 GHz (under 9.2 GHz), and τ_f =−43.0 ppm/°C, respectively.