The Enhanced Thermal Stability of (Mg_0.95Ni_0.05)₂TiO₄ Dielectric Ceramics Modified by a Multi-Phase Method

Abstract

The thermal stability of (Mg_0.95Ni_0.05)₂TiO₄ dielectric ceramics has been improved by mixing with CaTiO₃ phases owing to higher positive temperature coefficients. The pure (Mg_0.95Ni_0.05)₂TiO₄ and the mixture phase systems of CaTiO₃-modified (Mg_0.95Ni_0.05)₂TiO₄ were verified by XRD diffraction patterns to ensure the crystallite of different phases. The microstructures of the CaTiO₃-modified (Mg_0.95Ni_0.05)₂TiO₄ were observed by SEM and EDS to investigate the relation between element ratios and grains. As a result, it can be seen that the thermal stability of the CaTiO₃-modified (Mg_0.95Ni_0.05)₂TiO₄ can be effectively enhanced, compared with the pure (Mg_0.95Ni_0.05)₂TiO₄. Moreover, the radio frequency dielectric performances of CaTiO₃-modified (Mg_0.95Ni_0.05)₂TiO₄ dielectric ceramics are strongly dependent upon the density and the morphology of the specimens. The champion sample with the ratio of (Mg_0.95Ni_0.05)₂TiO₄ and CaTiO₃ of 0.92:0.08 showed an ε_r value of 19.2, an Qf value of 108,200 GHz, and a τ_f value of −4.8 ppm/°C, which may encourage (Mg_0.95Ni_0.05)₂TiO₄ ceramics to broaden the range of novel applications and match the requirements of 5G or next-generation communication systems.

1. Introduction

As a result of the continuous evolution of science and technology, millimeter-wave technology has been extensively developed and considered as a branch of 5G communication or next-generation mobile communication technology. Due to a large amount of data transmission, a multilayer board is required as the main circuit board design, leading to materials which should possess both low dielectric loss and conductor loss characteristics [1,2,3,4,5]. Dielectric materials, to date, are a good choice for creating high-frequency components that have a higher dielectric constant (ε_r), which can contribute to the miniaturization of components; high-quality factors (Qf values) can improve the energy of stored electromagnetic waves and the temperature coefficient of the resonance frequency (τ_f) approaching zero, which can enhance the thermal stability of the components. For example, when a filter possesses the three abovementioned characteristics, the filter results in effective downsizing, lower dielectric loss rates, greater filtering activity, and greater stability, unaffected by the external ambient temperature [6,7].

The ceramic systems based on MgTiO₃ have always received considerable attention and applications in the literature. For example, dielectric passive components of MgTiO₃ in a communication system have been employed to be a resonator, duplex, filter, and antenna [8,9,10,11,12]. Therefore, it is considerably important to enhance the dielectric performance of MgTiO₃. When the ratio of Mg and Ti is 2:1, the binary titanate ceramic Mg₂TiO₄ possesses a spinel-type structure that belongs to the cubic phase with Fd-3m space group (2 2 7). It demonstrates an ε_r value of 14, a Qf value of 150,000 GHz, and a τ_f value of −50 ppm/°C [13,14,15]. In addition, to further improve the dielectric characteristics, the method of partial substitution that selects Ni²⁺ (0.069 nm) with a radius resembling Mg²⁺ (0.078 nm) to perform the substitution was presented [16]. When the Mg²⁺ ions were replaced by Ni²⁺ ions to form (Mg, Ni)₂TiO₄ compositions, the (Mg_0.95Ni_0.05)₂TiO₄ compositions possessing a spinel-type structure presented a good dielectric performance with an ε_r value of 16.43, a Qf value of 238,000 GHz, and a τ_f value of −55 ppm/°C [17]. Moreover, an inferior (Mg_0.95Ni_0.05)TiO₃ phase also appeared during the synthesizing of (Mg_0.95Ni_0.05)₂TiO₄, which may be attributed to the effect of the thermal decomposition mechanism [18]. However, the slight (Mg_0.95Ni_0.05)TiO₃-doped (Mg_0.95Ni_0.05)₂TiO₄ possess a comparable performance, with an ε_r value of 17.2, a Qf value of 180,000 GHz, and a τ_f value of −45 ppm/°C [19], compared to those of the pure (Mg_0.95Ni_0.05)₂TiO₄ composition [17].

On the other hand, a multi-phase method has been employed to modify the dielectric characteristics. In Mg₂TiO₄–SrTiO₃ systems [20], Mg₂TiO₄ and SrTiO₃ have spinal cubic (lattice parameters: a = 8.439 Å, space group Fd3m) (ICDD-PDF#00-003-0858) and cubic perovskite (ICDD-PDF#01-084-0443), respectively. When x increased, the peak intensity of SrTiO₃ increased and the lattice parameters of Mg₂TiO₄ remained unchanged, which demonstrates that the two-phase system was used to modify relative permittivity in the Mg₂TiO₄–SrTiO₃ system. In (Mg_0.95Zn_0.05)₂TiO₄–SrTiO₃ systems [21], the lattice parameters had slight influence and remained unchanged after the SrTiO₃ was added into (Mg_0.95Zn_0.05)₂TiO₄, which confirms that the presence of a two-phase system could effectively promote densification in the (Mg_0.95Zn_0.05)₂TiO₄ matrix. In addition, CaTiO₃- and SrTiO₃-modified (Mg_0.95Co_0.05)₂TiO₄ ceramics [22,23] were presented using a multi-phase method for low–loss dielectric properties at microwave frequencies. Furthermore, thermal stability of dielectric ceramics is another important factor in practical applications. However, few studies have been conducted to discuss the thermal stability of (Mg_0.95Ni_0.05)₂TiO₄. For this purpose, we have made great efforts to enhance the thermal stability factor of (Mg_0.95Ni_0.05)₂TiO₄ compositions using a perovskite structured SrTiO₃ additive to form a two-phase system in our previous report [24].

In this study, we demonstrate the microstructure and radio frequency dielectric characteristics of the CaTiO₃-modified (Mg_0.95Ni_0.05)₂TiO₄ ceramic system for enhancing its thermal stability factor using the multi-phase method. We attempted to add CaTiO₃ with a high positive temperature coefficient to form a three-phase system and to compensate the negative temperature coefficient of (Mg_0.95Ni_0.05)₂TiO₄. X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive X-ray spectrometer (EDS) analyses were also employed to study the microstructure grain boundary, and compositions of the ceramic system. The radio frequency dielectric performances of the CaTiO₃-modified (Mg_0.95Ni_0.05)₂TiO₄ were quantified and examined by employing the formation of muti-phase coexistence.

2. Materials and Method

In this study, (Mg_0.95Ni_0.05)₂TiO₄ and CaTiO₃ were produced by a mixed-oxide solid-state reaction using the following high-purity chemical powders: magnesium oxide (MgO), nickel oxide (NiO), calcium carbonate (CaCO₃), and titanium dioxide (TiO₂). Because MgO is hygroscopic in nature, it first removes the moisture remaining at 600 °C for 2 h. The mixture of the above oxides was mixed according to the stoichiometries of (Mg_0.95Ni_0.05)₂TiO₄ and CaTiO₃. They were then ball ground in distilled water (ball grinding medium) for 24 h. All mixed powders were parched in a kiln and pre-phased (calcine) at 1100 °C for 4 h in a high-temperature furnace. Then, the pre-phased reagents were mixed again according to the chemical molar ratio of (1–x) (Mg_0.95Ni_0.05)₂TiO₄∙xCaTiO₃, and ball ground into a fine powder for 24 h. Then, polyvinyl alcohol (PVA 500; Showa, Tokyo, Japan) as a binder was added to the calcined powder, uniformly granulated, screened with a 100-mesh screen, and compressed with a pressure of 200 MPa to form a tablet form with a height of 0.5 cm and diameter of 1.1 cm. The sintering temperatures of the tablets were set at 1300~1425 °C for 4 h in air. The risen and dropped temperature rates were set at 10 °C/min for all samples.

The crystallization-phase observation of the pre-phased powder and mixture compositions were analyzed by XRD (Rigaku D/Max III. V., Tx for USA) The lattice constant was calculated using the Rietveld method to fit the XRD patterns. SEM (Philips XL-40FEG, Eindhoven, The Netherlands) was employed to observe the surface morphologies of samples, and EDS was utilized to demonstrate the different phases and compositions. The apparent densities of the samples were measured using the Archimedes method. The ε_r and Qf values at radio frequencies were measured by the Hakki–Coleman dielectric resonator method [25,26,27].

The measurement was mainly composed of a vector network analyzer (HP8757D, Agilent Technologies, Taipei, Taiwan) and sweep oscillator connections (HP8350B, Agilent Technologies, Taipei, Taiwan). The thermal stability (${\tau }_f$ values) was evaluated with a temperature range from 20 to 80 °C. The following Formula (1) was utilized to obtain the τ_f value (ppm/°C):

$${\tau }_f=\frac{f_2-f_1}{f_1\left(T_2-T_1\right)}$$

(1)

where f₁ and f₂ represent the resonant frequencies at T₁ and T₂, respectively.

3. Results and Discussion

Figure 1 presents the XRD patterns of the (1–x) (Mg_0.95Ni_0.05)₂TiO₄∙xCaTiO₃ with an x value of 0.08 after sintering at different temperatures for 4 h. According to the JCPDS card, a three-phase system can be observed, which consisted of (Mg_0.95Ni_0.05)₂TiO₄ phase with a spinel-type structure as the primary crystalline phase (Mg₂TiO₄, ICDD–PDF#00-025-1157; lattice constants a = b = c = 0.84409 nm), (Mg_0.95Ni_0.05)TiO₃ phase (MgTiO₃, ICDD–PDF#00-006-0494), and a CaTiO₃ phase with perovskite structure (JCPDS #22-0153). The lattice constant was slightly decreased with the increasing sintering temperature (a = b = c from 0.84005 to 0.83456 nm), indicating sharper main peaks of the (Mg_0.95Ni_0.05)₂TiO₄ phase and larger grain size. No other obvious phases and impurities were identified in Figure 1. In addition, the reason for the presence of the (Mg_0.95Ni_0.05)TiO₃ phase is generally considered to be the uneven particle size of the initial materials, which increases the probability of secondary crystallization nucleation. Another reason for the formation of the (Mg_0.95Ni_0.05)TiO₃ phase may be the effect of the thermal decomposition mechanism [18]. However, the ratio of (Mg_0.95Ni_0.05)TiO₃ phase in the ceramics was decreased due to the increased grain boundary motion with the increasing sintering temperature, as shown in

Table 1. The lattice parameter and compositions of 0.92∙(Mg_0.95Ni_0.05)₂TiO₄ 0.08CaTiO₃ sintered at different temperatures for 4 h. (Mg_0.95Ni_0.05)TiO₃ and CaTiO₃ ratios are determined by all peak areas of each phase.

Temperatures	a = b = c (nm)	(Mg0.95Ni0.05)TiO3 Ratio (%)	CaTiO3 Ratio (%)
1300	0.84005 ± 0.1033	17.4	11.6
1325	0.84005 ± 0.1033	17.2	13.4
1350	0.83986 ± 0.0972	17	13.5
1375	0.83456 ± 0.0995	14.9	13.8
1400	0.83456 ± 0.0995	14.7	13.8
1425	0.83456 ± 0.0995	14.4	14.1

. Moreover, the (Mg_0.95Ni_0.05)TiO₃ had no noticeable impact on the dielectric properties of (Mg_0.95Ni_0.05)₂TiO₄ in our previous study [19]. Therefore, the influence of the inferior (Mg_0.95Ni_0.05)TiO₃ phase can be negligible when the sintering temperature is over 1350 °C.

Figure 2 presents the results of the XRD analysis of the (1–x) (Mg_0.95Ni_0.05)₂TiO₄ xCaTiO₃ with different x values. It can be observed that the different contents have no obvious influence on the phase growth of the (Mg_0.95Ni_0.05)₂TiO₄. A few (Mg_0.95Ni_0.05)TiO₃ peaks still existed in all patterns. As mentioned above, the effect of the inferior (Mg_0.95Ni_0.05)TiO₃ phase on dielectric performance is quite slight and can be neglected. The lattice constant of the (Mg_0.95Ni_0.05)₂TiO₄ with a different amount of the CaTiO₃ contents is presented in

Table 2. The lattice parameter and compositions of (1–x) (Mg_0.95Ni_0.05)₂TiO₄∙xCaTiO₃ sintered at 1350 °C for 4 h. (Mg_0.95Ni_0.05)TiO₃ and CaTiO₃ ratios are determined by all peak areas of each phase.

x Values	a = b = c (nm)	(Mg0.95Ni0.05)TiO4 Ratio (%)	CaTiO3 Ratio (%)
0.06	0.84005 ± 0.1033	17.1	9.6
0.08	0.84005 ± 0.1033	17	13.5
0.09	0.83986 ± 0.0972	14.5	13.5
0.1	0.83456 ± 0.0995	13.3	13.7
0.12	0.83456 ± 0.0995	14	14.4

. It was also observed that all samples have a spinal cubic structure with the lattice constants (a = b = c) from 0.84005 to 0.83456 nm, indicating that the (Mg_0.95Ni_0.05)₂TiO₄ is still in the primary phase. When CaTiO₃ was blended with (Mg_0.95Ni_0.05)₂TiO₄, no obvious influence on the lattice constants of (Mg_0.95Ni_0.05)₂TiO₄ could be found (Table 2). Furthermore, the growth of mixed phases in the one ceramic may cause a negative effect due to structural dissimilarities and the larger ionic radii values of Ca²⁺ (0.106 nm) compared to those of Mg²⁺ (0.078 nm) and Ni²⁺ (radii = 0.069 nm) [16]. However, the XRD analysis confirms the coexistence of the multiple phases without the structural dissimilarities in our samples.

Figure 3 shows the SEM images of the morphologies of the 0.92∙(Mg_0.95Ni_0.05)₂TiO₄∙0.08CaTiO₃ sintered at different temperatures. When the sintering temperature was increased, the grain size increased in Figure 3a–f, which is consistent with the XRD results (Figure 1). The pores reduced with the sintering temperature increasing from 1300 to 1350 °C. The reason for the increase in the grain size and the decrease in pores is the thermal drive energy, which enables connection and expands the neck between the grains. Therefore, Figure 3a–c exhibits the grain growth with better movement of the grain boundary and a more uniform and dense morphology. However, when the sintering temperature was increased from 1350 to 1425 °C, the grain size still increased, and the pore size became larger. This is attributed to the excessive extrusion between grains and air pressure under too-high sintering temperatures, which is similar with previous reports [20,21,22,23,24]. Furthermore, grains of 0.92∙(Mg_0.95Ni_0.05)₂TiO₄∙0.08CaTiO₃ can be roughly divided into three shapes, as shown in Figure 3c. The EDS results of each grain are summarized in

Table 3. Composition analysis from EDS results for spot A, B, and C in Figure 3c.

Atom (%)
Spot	Mg	Ni	Ca	Ti	O
A	17.54	2.2	0	16.84	63.42
B	24.27	2.25	0	19.51	53.97
C	0	0	20.42	19.49	60.09

. Therefore, the different grains were identified as follows: spot A is (Mg_0.95Ni_0.05)TiO₃; spot B is (Mg_0.95Ni_0.05)₂TiO₄’ and spot C is CaTiO₃. The EDS results are consistent with the XRD analysis, verifying that the (1–x) (Mg_0.95Ni_0.05)₂TiO₄∙xCaTiO₃ is a three-phase coexistence system.

Figure 4 shows the results of the apparent density and dielectric constant (ε_r values) of the (1–x) (Mg_0.95Ni_0.05)₂TiO₄∙xCaTiO₃ with different x values and temperatures. The apparent density was increased with the increasing x value because CaTiO₃ (~4.036 g/cm³) possesses a higher density than that of (Mg_0.95Ni_0.05)₂TiO₄ (~3.49 g/cm³) [17]. Moreover, the apparent density value increased when the temperature increased from 1300 to 1350 °C, which is due to the grain growth (Figure 3a–c). However, the apparent density was reduced due to the enlarged pore size (Figure 3d–f) when the temperature increased from 1350 to 1425 °C. These results lead to the optimal sintering temperature of 1350 °C being obtained. Due to much higher dielectric constant (ε_r) of (Mg_0.95Ni_0.05)₂TiO₄ (~16.4) and CaTiO₃ (~170) than that of air (~1), the ε_r of the (1–x) (Mg_0.95Ni_0.05)₂TiO₄ xCaTiO₃ correlates positively with apparent density. Therefore, the ε_r changed with the change in the apparent density and the x value. Therefore, the dielectric constant changes from 18.6 to 20.9 as x increases from 0.06 to 0.12, as shown in

Table 4. Microwave dielectric performances of (1–x) (Mg_0.95Ni_0.05)₂TiO₄ xCaTiO₃ at 1350 °C for 4 h.

x Values	Density (g/cm3)	εr Values	Qf Values (GHz)	τf Values (ppm/°C)
0.06	3.44	18.6	131,000	−20.7
0.08	3.48	19.2	108,200	−4.8
0.09	3.51	19.6	90,000	1.3
0.1	3.53	20.1	75,000	13.9
0.12	3.56	20.9	58,000	30.5

.

Figure 5 shows that the Qf value (quality factor) of the (1–x) (Mg_0.95Ni_0.05)₂TiO₄ xCaTiO₃ decreased with the increasing the x value. It is attributed to the fact that the Qf value of CaTiO₃ (~3600 GHz) was relatively smaller than that of (Mg_0.95Ni_0.05)₂TiO₄ (~238,000 GHz). In addition, the Qf of the (1–x) (Mg_0.95Ni_0.05)₂TiO₄∙xCaTiO₃ rises at 1300–1350 °C and drops at 1350–1425 °C in Figure 5. Compared with Figure 4, the Qf of the (1–x) (Mg_0.95Ni_0.05)₂TiO₄ xCaTiO₃ correlated positively with the ε_r and apparent density. Although there are many factors that impact the Qf, such as dielectric loss (which is caused by the second phase), oxygen vacancy, grain size, and porosity [28], the Qf is mainly dependent on the apparent density in our case. Similar with the results of the apparent density at different temperatures (Figure 4), the Qf also reached the optimal value at 1350 °C, as listed in Table 4. The τ_f of the (1–x) (Mg_0.95Ni_0.05)₂TiO₄∙xCaTiO₃ sintered at different temperatures seems to maintain a constant value, which is strongly dependent on the CaTiO₃ contents (x value), as shown in Figure 5 and Table 4. The composition and phase determined the thermal stability of the (1–x) (Mg_0.95Ni_0.05)₂TiO₄∙xCaTiO₃ because a high positive temperature coefficient of CaTiO₃ can compensate the negative temperature coefficient of (Mg_0.95Ni_0.05)₂TiO₄. Therefore, the τ_f was changed from −20.7 to 30.5 (ppm/°C) when x increased from 0.04 to 0.12, indicating that τ_f = 0 is possible after controlling x. It is worth noting that the τ_f was only −4.8 ppm/°C at x = 0.08 but the Qf was kept at 108,200 GHz.

4. Conclusions

In conclusion, the thermal stability of the CaTiO₃-modified (Mg_0.95Ni_0.05)₂TiO₄ ceramic system was investigated. The dense morphology without pores can be obtained at 1350 °C, resulting in the optimal sintering temperature of the apparent density, ε_r, and Qf. A high positive temperature coefficient of CaTiO₃ can be used to improve the thermal stability of (Mg_0.95Ni_0.05)₂TiO₄. The τ_f can be adjusted with changing the CaTiO₃ contents, even closing to zero. The excellent dielectric characteristics of the 0.92∙(Mg_0.95Ni_0.05)₂TiO₄∙0.08CaTiO₃ sintered at 1350 °C was presented with an ε_r of 19.2, an Qf of 108,200 GHz, and a τ_f of −4.8 ppm/°C. Therefore, the CaTiO₃-modified (Mg_0.95Ni_0.05)₂TiO₄ ceramic system showed high thermal stability and performance, suggesting the potential of these ceramics as dielectric substrate materials and radio frequency passive components in the microwave field to miniaturize components and transmit signals neglecting the temperature factor.