Sintering Behavior, Microstructure and Microwave Dielectric Properties of Li₂TiO₃-Based Solid Solution Ceramics with Lithium Fluoride Addition for Low-Temperature Co-Fired Ceramic Applications

Abstract

Nowadays, low-temperature co-fired ceramic (LTCC) technology has become one of the main forms of manufacturing electronic devices. However, a majority of microwave dielectric ceramics are not suitable as LTCC materials due to their high sintering temperatures. Developing novel LTCC materials with good microwave dielectric properties is extremely urgent. In this paper, an LiF sintering aid was added to Li₂Ti_0.8(Co_1/3Nb_2/3)_0.2O₃ (LTCN) ceramics to explore new LTCC materials. The sintering behavior, microstructure and microwave dielectric properties of LTCN + x wt% LiF ceramics were investigated in detail. The results indicated that the addition of LiF increased the degree of disorder in the LTCN matrix, transforming it from a monoclinic to a cubic crystal system. The ceramics exhibited relatively dense and homogeneous microstructures at the sintering temperature of 950 °C as the LiF doping amount was not less than 2 wt%. By LiF doping, the quality factor (Q × f) value was significantly enhanced due to the improved microstructure. Meanwhile, the temperature coefficient of resonant frequency (τ_f) of LTCN ceramics was successfully regulated to the near zero value owing to the negative τ_f characteristic of LiF. Excellent microwave dielectric properties of dielectric constant (ε_r) = 19.01, Q × f = 144,890 GHz, τ_f = −1.52 ppm/°C were obtained when the sample doped 3 wt% LiF was sintered at 950 °C for 3 h. Furthermore, the good chemical compatibility of the LTCN-3 wt% LiF ceramic with silver electrodes suggested that the ceramic was a potential material for LTCC applications.

1. Introduction

In the material family, oxides have attracted much attention due to their excellent mechanical properties, physical properties, and chemical properties [1,2] and, therefore, have been widely used in the field of optoelectronics, catalysis and electronics. In recent decades, the rapid development of wireless communications has made microwave dielectric ceramic devices with oxide components, such as duplexers, dielectric filters and multiplexers, a new research hotspot [3,4,5]. Moreover, low-temperature co-fired ceramic (LTCC) technology, which occupies a crucial role in the integration and miniaturization of multilayer devices, has become one of the dominant forms of fabricating electronic devices, and has raised the requirements for microwave dielectric ceramics. In addition to excellent dielectric properties, LTCCs also require relatively low sintering temperatures (below the melting point of metal electrode) to ensure co-fire with metal electrodes such as silver (Ag) [6,7,8,9]. However, the majority of microwave dielectric ceramics have high sintering temperatures (>1000 °C) which are not suitable for LTCC applications. As a result, the development of new microwave ceramics with good dielectric properties and low sintering temperatures is extremely urgent.

Lithium (Li)-based rock salt materials have been widely investigated as candidate materials for microwave applications [5,10,11,12]. Among them, lithium metatitanate (Li₂TiO₃) is an excellent candidate due to its suitable dielectric constant and cheap raw materials [6]. However, its Q × f (Q = tanδ⁻¹, tanδ and f refer to dielectric loss and resonant frequency, respectively) value (63,500 GHz) is relatively low due to Li volatilization caused by an excessively high sintering temperature and microstructure deterioration caused by an order–disorder phase transition [13]. To resolve this problem, the oxide doping method is generally used. For instance, Bian et al. [14] revealed that a small amount of magnesium oxide (MgO) doping could improve the Q × f value by lowering the ordering degree of Li₂TiO₃. Zhang et al. [15] revealed that the Q × f value of Li₂TiO₃ could be enhanced by increasing the packing fraction by doping small amounts of nickel oxide (NiO). However, doping single oxides to the ceramic matrix will not always work in the improvement of the τ_f value [16]. Recent studies have shown that complex ion doping can modulate the τ_f of Li₂TiO₃ but can improve the Q × f value. The replacement of Ti⁴⁺ ions with (Mg_1/3Nb_2/3)⁴⁺ ions not only inhibits the formation of cracks in the ceramic but also suppresses the reduction in Ti⁴⁺ ions and the diffusion of Li⁺ ions, successfully increasing the Q × f value of Li₂TiO₃ from 63,500 GHz to 113,774 GHz, and adjusting the τ_f value from +20.3 ppm/°C to +13.38 ppm/°C [17]. Chen et al. and Zhang et al. have also made good progress in the study of (Co_1/3Nb_2/3)⁴⁺ and (Al_0.5Nb_0.5)⁴⁺ complex ions replacing Ti⁴⁺ ions, respectively [18,19]. Shortly afterwards, Zhan et al. developed novel microstrip antenna with remarkable wide temperature stability for millimeter wave applications by introducing 10.8 mol% LMTZN into the Li₂TiO₃ system [20]. However, the issue of the high sintering temperatures of Li₂TiO₃-based ceramics still does not have a suitable solution.

As we know, a common way of lowering the sintering temperature of Li₂TiO₃-based ceramics is adding glasses or oxides with a low melting point. For example, Chen et al. [20] used Li₂O-ZnO-B₂O₃ to decrease the sintering temperature of Li₂TiO₃ ceramics to 900 °C. Guo et al. [21] utilized boric acid (H₃BO₃) to lower the sintering temperature of Li₂Ti_0.8(Cu_1/3Nb_2/3)_0.2O₃ from 1140 °C to 860 °C. Li₂Ti_0.75(Mg_1/3Nb_2/3)_0.25O₃ ceramics with a near-zero τ_f value (+4.3 ppm/°C and −6.1 ppm/°C) and low sintering temperature (910 °C and 870 °C) have been successfully obtained after doping 2 wt% vanadium pentoxide (V₂O₅) and 1 wt% 0.6CuO–0.4B₂O₃ additions, respectively [22]. A sintering temperature below 950 °C (~790 °C) has been also obtained in a 1 wt% CuO–B₂O₃ doped Li₂Ti_0.98Mg_0.02O_2.96F_0.04–1 wt% niobium pentoxide (Nb₂O₅) ceramic system [23]. However, these low-temperature sintering aids, especially glass materials, would inevitably deteriorate the Q × f value of the matrix because of higher dielectric losses. In contrast, lithium fluoride (LiF), which has the same rock salt structure as Li₂TiO₃, exhibits a low sintering temperature (800 °C) and a high Q × f (78,800 GHz), making it well suited as a low-temperature sintering aid [24,25,26,27]. For example, Hao et al. [28] reduced the sintering temperature to 950 °C by adding 2.5 wt% LiF to Li₂TiO₃. Meanwhile, the Q × f of Li₂TiO₃ was improved because of the enhancement in the microstructure. The sintering temperatures of the fluorine oxides Li₅Ti₂O₆F and Li₇Ti₃O₉F were below 950 °C, and the Q × f values were also greatly improved [29,30]. In our previous work, the sintering temperature of Li₂Ti_0.9(Zn_1/3Ta_2/3)_0.1O₃ was reduced to 950 °C using LiF as sintering aid, and a high Q × f value (110,090 GHz) was also obtained [31]. Therefore, it is a viable solution to use LiF as a low-temperature sintering aid for Li₂TiO₃-based ceramics.

On the one hand, it is obvious from

Table 1. Microwave dielectric properties of some Li₂TiO₃-based ceramics.

Ceramic System	εr	Q × f (GHz)	τf (ppm/°C)	Reference
0.8Li2TiO3-0.2MgO	19.2	106,226	+3.56	[14]
0.8Li2TiO3-0.2NiO	20.4	83,608	+1.97	[16]
0.7Li2TiO3-0.3ZnO	22.95	99,800	−32.7	[12]
Li2Ti0.8(Cu1/3Nb2/3)0.2O3	18.3	77,840	+9.8	[21]
Li2Ti0.85(Zn1/3Ta2/3)0.15O3	18.69	102,531	+11.8	[32]
Li2Ti0.85(Mg1/3Ta2/3)0.15O3	19.48	80,005	+9.5	[33]
Li2Ti0.7(Co1/3Nb2/3)0.3O3	21.3	110,000	0	[18]
Li2Ti0.7(Al1/3Nb2/3)0.3O3	21.2	181,800	+12.8	[19]
Li2Ti0.7(Mg1/3Nb2/3)0.3O3	19.01	113,774	+13.38	[17]
Li2Ti0.8(Co1/3Nb2/3)0.2O3	18.83	102,500	+9.27	This work

that the Li₂Ti_0.8(Co_1/3Nb_2/3)_0.2O₃ ceramic has a higher Q × f compared with the others. On the other hand, the Li₂Ti_0.8(Co_1/3Nb_2/3)_0.2O₃ with a positive τ_f value can be mixed with LiF to obtain a ceramic material with a near-zero τ_f value. Meanwhile, the sintering temperature of LTCN ceramics can be effectively decreased due to the low melting point (848 °C) of LiF. As far as we know, the effects of LiF additions on the sintering behavior, microstructure and microwave dielectric properties of Li₂Ti_0.8(Co_1/3Nb_2/3)_0.2O₃ solid-solution ceramics have not been reported. For such a purpose, Li₂Ti_0.8(Co_1/3Nb_2/3)_0.2O₃-x wt% LiF (x = 1–5) ceramics were prepared by the solid-state reaction method in this paper. As expected, a novel microwave dielectric material with a low sintering temperature, high Q × f and near-zero τ_f was successfully obtained. Furthermore, the sintering properties, microwave dielectric properties and chemical compatibility of the ceramics with silver electrodes were investigated in detail.

2. Materials and Methods

2.1. Materials

Lithium carbonate (Li₂CO₃, 99.99%), titanium oxide (TiO₂, 99%), cobalt monoxide (CoO, 99%), niobium oxide (Nb₂O₅, 99.9%) and lithium fluoride (LiF, 99.99%) were provided by Aladdin Biochemical Technology Co., Ltd. (Aladdin Chemical Reagent Company, Shanghai, China).

2.2. Fabrication of LTCN + x wt% LiF Ceramics

Traditional solid-state reaction method was used to synthesize the LTCN + x wt% LiF ceramics. After drying in an oven, all raw materials were weighed stoichiometrically. The LTCN pre-sintered powder was obtained by wet milling in a nylon jar for 8 h using alcohol as solvent and yttria-stabilized tetragonal zirconia polycrystals (Y-TZP) balls as ball-milling medium. Subsequently, the slurry was dried at 80 °C for 4 h and calcined at 850 °C for 2 h to obtain the LTCN powder. The LTCN powder was crushed and 1–5 wt% LiF was added to it, respectively, and then ball milled again for 8 h. After drying the ball-milled powder, a 5 wt% concentration of polyvinyl alcohol (PVA) solution was added to increase its adhesion. The prefabricated powder was pressed into cylinders with a diameter and thickness of 10 mm and 5 mm, respectively. The cylinders were placed in an alumina crucible with a lid and covered with a powder of the same composition (to prevent evaporation of the volatile elements, lithium, cobalt and fluorine), heated to 600 °C for 2 h to expel PVA and sintered at 900–1000 °C for 3 h. Finally, to investigate the chemical compatibility of the LTCN + x wt% LiF ceramics, 20 wt% Ag powder was added to the ceramic powder, the mixture was pressed into sheets and then sintered at 950 °C for 3 h.

2.3. Structural and Properties Characterizations

The bulk density of the samples was determined by the Archimedes method (XS64, Mettler Toledo, USA) with a medium of deionized water. Analysis of the phase structure of the samples was carried out by X-ray diffractometry (Ultima IV, Rigaku Corporation, Showashima, Tokyo, Japan) operating at 30 mA and 45 kV with Cu Kα radiation in the range of 2θ angles from 15° to 75°. The microscopic morphology of the samples was observed by scanning electron microscopy (SEM, Mira3, Tescan, Czech) at an accelerating voltage of 30 kV and the elemental composition of the samples was analyzed by energy dispersive X-ray spectroscopy (EDS, AztecOne, Oxford Instruments, London, UK). The ε_r and Q × f values in microwave frequencies ranging from 8.0 to 12.0 GHz were measured by the Hakki–Coleman dielectric resonator method and cavity approach using vector network analyzer (N5230C, Agilent, Palo Alto, California, USA), respectively. The value of τ_f was calculated from the resonant frequencies f₂₅ and f₈₅ measured at 25 °C and 85 °C by Equation (1).

$${\tau }_f=\frac{(f_{85}-f_{25})}{60\times f_{25}}\times {10}^6 (\mathrm{p}\mathrm{p}\mathrm{m}/°\mathrm{C})$$

(1)

3. Results and Discussion

3.1. Phase Composition and Structure Analysis

The X-ray diffraction (XRD) patterns of LTCN + x wt% LiF (x = 0–5) ceramics sintered at 950 °C for 3 h are shown in Figure 1a. Meanwhile, the XRD pattern of the pure LTCN ceramic is presented in Figure 1 as comparison. It is obvious from Figure 1a that the transition from an ordered monoclinic phase to a disordered cubic phase occurs after LiF doping. That is, all LTCN + x wt% LiF ceramics present pure cubic Li₂TiO₃ phases (PDF # 03-1024). As shown in Figure 1b, the (200) peak shifts to a higher 2θ direction with increasing LiF content. This phenomenon should be attributed to the variation in lattice parameters according to the Bragg equation (Equation (2)):

$$n\lambda =2dsin\theta$$

(2)

where d, θ, λ and n are the crystal spacing, Bragg angle, X-ray wavelength and number of reflection levels, respectively. The general formula for LTCN + x wt% LiF ceramics can be expressed as Li_{(2−x)/(3−2x)}Ti_{(0.8−0.8x)/(3−2x)}Co_{(0.2/3−0.2x/3)(3−2x)}Nb_{(0.4/3−0.4x/3)(3−2x)}O_{(3−3x)/(3−2x)}F_x/(3−2x) based on the MgO-type rock salt structure. Therefore, the [Li_2/3Ti_0.8/3(Co_1/3Nb_2/3)_0.2/3]²⁺ ion with an effective ionic radius of 0.71 Å is replaced by the Li⁺ ion with an effective ionic radius of 0.76 Å and the O²⁻ ion with an effective ionic radius of 1.4 Å is substituted by the F⁻ with an effective radius of 1.33 Å [34]. The substitution of cations increases the ionic radius by 0.05 Å and the replacement of anions decreases the ionic radius by 0.07 Å, which indicates a decrease in the net ionic radius of 0.02 Å. This type of substitution leads to a contraction of the lattice and a smaller spacing of the crystal planes, causing the diffraction peaks of the X-rays to be shifted to a higher angle. The calculated cell volumes of LTCN + x wt% LiF (x = 0–5) ceramics (see

Table 2. Lattice parameters and cell volumes of LTCN + x wt% LiF ceramics.

x Value	Lattice Parameter (a = b = c) (Å)	Cell Volume (Å3)
1	4.1518(8)	71.570(9)
2	4.1516(7)	71.559(7)
3	4.1504(1)	71.495(0)
4	4.1495(4)	71.449(9)
5	4.1488(7)	71.415(1)

) also further confirm this phenomenon. Our result is consistent with the phenomenon in a LiF-doped Li₂TiO₃ system reported by Ding et al. [35].

3.2. Density and Microscopic Morphology Analysis

Figure 2a shows the bulk density of LTCN + x wt% LiF ceramics (x = 1–5) sintered at different temperatures for 3 h. When adding 1 wt% LiF to LTCN ceramics, the density keeps increasing with the sintering temperature and still fails to densify at 1000 °C. When the doping amount rises to 2 wt%, the density rises significantly and the densification temperature of the ceramics is stable at 950 °C. This variation indicates that the doping of LiF has an outstanding effect on the sintering behavior of LTCN ceramics and successfully reduces the densification temperature of LTCN ceramics from 1300 °C to 950 °C. Figure 2b shows the diameter shrinkage of LTCN ceramic sintered samples at 950 °C for different LiF doping amounts. As the LiF doping amount increases from 1 wt% to 5 wt%, the diameter shrinkage shows an increasing and then decreasing trend, reaching a maximum value of 11% at x = 4. The apparent density shows a similar trend with the x value and reaches a saturation value of about 3.07 g/cm³ at x = 4. The maximum density of the LTCN + 4 wt% LiF sample is higher than the density (< 2.97 g/cm³) of Li₂TiO₃ that was synthesized from nano powders [36]. The increasing bulk density of LiF doping at 1–4 wt% can be attributed to the accelerated mass transport along the boundary by the formation of a liquid phase of LiF during the sintering process. When the LiF content is up to 5 wt%, the excess liquid phase can form too thick a film between the grains, which is counterproductive to mass transfer. That is, LiF mainly promotes the sintering performance of LTCN ceramics in two ways: On the one hand, it replaces O with F with a smaller effective ionic radius to weaken the oxygen bond strength and, hence, the intrinsic sintering temperature is reduced. On the other hand, it forms a liquid phase (with a melting point of 845 °C) during the sintering process to enhance the grain boundary mass transfer [28].

Figure 3 shows the SEM images and grain size distributions of LTCN ceramics sintered at 1300 °C for 3 h and LTCN ceramics doped with 1–5 wt% LiF sintered at 950 °C for 3 h, respectively. The grain sizes in the insets were obtained by measuring the grain size of at least 100 grains according to the linear intercept method using the Nano Measurer software. A significant increase in pores can be found after doping 1 wt% LiF into the LTCN ceramic, revealing that 1 wt% LiF is insufficient for densifying the ceramics at lower sintering temperatures, e.g., 950 °C. When the LiF doping amount is greater than 2 wt%, the microstructure of the samples becomes denser and clearer grain boundaries and a more uniform grain can be found. Figure 3a–f shows that a significant decrease in the average grain size [from 23.1 μm to (2.8–7.2) μm] occurs when the LiF is introduced due to the declined sintering temperature. In general, the higher the sintering temperature, the larger the average grain size for a given material composition. In addition, the average grain size increases with the increase in LiF content, increasing from 2.8 μm at x = 2 to 7.2 μm at x = 5. This can be explained as large liquid phase accelerates the grain growth. A similar phenomenon has been reported in Li₂MgTi_0.7(Mg_1/3Nb_2/3)_0.3O₄-doped LiF ceramics [37]. As a result, a moderate amount of LiF can significantly improve the microstructure of LTCN ceramics.

The SEM images of 3 wt% LiF-doped ceramics sintered at 950 °C for 3 h and corresponding EDS element mapping results, except the Li element, are shown in Figure 4. It is evident that the oxygen (O), titanium (Ti), cobalt (Co), niobium (Nb) and fluorine (F) are uniformly distributed throughout the region and no segregation is observed in any region. The proportions of the elements in the inset of Figure 4g are close to the stoichiometric ratio.

3.3. Microwave Dielectric Properties Analysis

Figure 5a,b shows the ε_r and Q × f values of LTCN + x wt% LiF ceramics (x = 1–5) sintered at 900–1000 °C for 3 h, respectively. As can be seen, the trend of the dielectric constant as a function of the sintering temperature is consistent with the variation in the density in Figure 2, showing that the density is the main factor affecting the dielectric constant. A high density means that the number of pores with a low ε_r value (ε_r,air ≈ 1) is reduced. However, the variation in the dielectric constant for different LiF doping amounts does not strictly follow the law of density variation. It is well known that in addition to density and phase composition, dielectric polarizability is a major influence on the dielectric constant of ceramics [38,39]. Here, the dielectric polarization rates of Li⁺ (1.20 Å) and F⁻ (1.62 Å) are lower than [Ti_0.8(Co_1/3Nb_2/3)_0.2]⁴⁺ (2.98 Å) and O²⁻ (2.01 Å), respectively [40]. That is, the ε_r value should decrease gradually with the increasing LiF doping amount if the effect of density on the ε_r value can be ignored. Therefore, as shown by the dependence of ε_r on x in Figure 4a, it can be seen that the ε_r value of LTCN + x wt% LiF ceramics is determined by the density and dielectric polarizability.

It has been well documented that quality factors are closely related to density, grain size, second phase and crystal structure [41,42,43]. In this study, the trend of the Q × f value with sintering temperature is similar to that of the ε_r value. However, the Q × f value is not maximized at x = 4, which is the case of the density, but reaches a maximum value of 144,890 GHz at x = 3. This suggests that many factors other than density also have a significant effect on the Q × f value of LTCN + x wt% LiF ceramics. Since the second phase does not exist in LTCN + x wt% LiF ceramics, the effects of grain size and the amount of LiF doping are focused on. At x ≤ 3, the increase in density and grain size plays a significant role in the improvement of the Q × f value. Previous studies have shown that the excessive addition of LiF has a significant negative effect on Q × f [31]. Therefore, the excess LiF causes a decreasing trend in the Q × f values of the ceramics as x > 3 [44]. However, the effect of all these factors on Q × f is difficult to quantify.

Figure 6 shows the variation in the τ_f value of LTCN + x wt% LiF ceramics sintered at 950 °C for 3 h. It is clearly seen that the τ_f value gradually decreases with the increase in LiF doping amount, mainly due to the moderating effect of LiF with a negative τ_f value (−135 ppm/°C) on LTCN with a positive τ_f value (+9.27 ppm/°C) [45,46]. In this study, the near-zero τ_f values are achieved at 2 ≤ x ≤ 4, although the LiF doping amount has a significant effect on the Q × f values (Figure 5b). Therefore, the balance between the τ_f value and the Q × f value is important for practical applications of microwave dielectric ceramics. In summary, the LTCN + 3 wt% LiF ceramic sintered at 950 °C for 3 h exhibits good microwave dielectric properties of ε_r = 19.01, Q × f = 144,890 GHz and τ_f = −1.52 ppm/°C.

Table 3. Comparison of sintering temperature and microwave dielectric properties of some ceramics with rock salt structures.

Ceramic System	ST. (°C)	εr	Q × f (GHz)	τf (ppm/°C)	Reference
Li2TiO3-2.5 wt%LiF	950	24.01	75500	+36.2	[28]
Li2Ti0.9(Zn1/3Ta2/3)0.1O3-3 wt%LiF	950	23.14	110,090	+3.25	[31]
0.9Li2TiO3-0.1LiF	1100	23.6	108,000	+4.2	[35]
Li2MgTi0.7(Mg1/3Nb2/3)0.3O4-3 wt% LiF	1100	16.32	145,384	−16.33	[37]
Li3TiO3F	900	17.28	96,280	−32.7	[47]
Li5Ti2O6F	880	19.6	79,500	−29.6	[29]
Li7Ti3O9F	950	22.5	88,200	−24.2	[30]
Li4Mg2NbO6F	900	15.53	93,300	−39.8	[48]
LTCN-3 wt%LiF	950	19.01	144,890	−1.52	This work

ST.: Sintering temperature.

provides a comparison of sintering temperatures and microwave dielectric properties of some ceramics with rock salt structures. Compared with other ceramics, the LTCN-3 wt% LiF ceramic has the best overall performance. Its Q × f value is significantly higher than that of the ceramics, except for the Li₂MgTi_0.7(Mg_1/3Nb_2/3)_0.3O₄-3 wt% LiF ceramic, as shown in Table 3. However, the sintering temperature of the Li₂MgTi_0.7(Mg_1/3Nb_2/3)_0.3O₄-3 wt% LiF ceramic is too high (1100 °C) to meet the requirements of LTCC technology. Therefore, the LTCN-3 wt%LiF ceramic is very competitive in rock salt structure compounds for LTCC applications because of its low sintering temperature and excellent microwave dielectric properties.

3.4. Chemical Compliance with Silver Electrode Analysis

The chemical compatibility is an important criterion to determine whether ceramics can be used as LTCC materials. To study the chemical compatibility of LTCN + x wt% LiF ceramics, 20 wt% Ag powder was mixed with LTCN + 3 wt% LiF ceramic powder, these powders were pressed into the cylinder and were sintered at 950 °C for 3 h. The results of XRD, SEM and EDS elemental analyses of LTCN + 3 wt% LiF co-fired with silver are shown in Figure 7. It is obvious from Figure 7a that the XRD pattern matches well with the diffraction peaks of c-Li₂TiO₃ (PDF # 03-1024) and Ag (PDF # 04-0783), and no impurity phase appears. From the inset of Figure 7a, the brighter A and the darker B are clearly separated. According to the EDS elemental analysis results of points A and B in Figure 6b, point A is the monolithic Ag and point B is the LTCN + 3 wt% LiF ceramic, implying that there is no chemical reaction between the ceramic and Ag. Hence, the LTCN + 3 wt% LiF ceramic is a promising material for LTCC applications.

4. Conclusions

In this paper, the phase structure, microstructure, microwave dielectric properties and chemical compatibility with Ag electrodes of LTCN + x wt% LiF ceramics were investigated. A continuous solid solution was formed throughout the entire range of LiF content. The addition of LiF transformed the LTCN matrix from an ordered monoclinic phase to a disordered cubic phase. By LiF doping, the sintering temperature of LTCN ceramics was effectively reduced to 950 °C and the quality factor (Q × f) value was significantly enhanced due to the improved microstructure. Meanwhile, the τ_f of LTCN ceramics was successfully regulated to the near zero value, owing to the negative τ_f of LiF. Excellent microwave dielectric properties of ε_r = 19.01, Q × f = 144,890 GHz, τ_f = −1.52 ppm/°C were achieved for the LTCN + 3 wt % LiF ceramic sintered at 950 °C for 3 h. The chemical compatibility results showed that there is no chemical reaction between the ceramic and Ag. All results indicate that the LTCN + 3 wt % LiF ceramic is a promising material for LTCC applications.