Effect of Y₂O₃-Al₂O₃ Additives on the Microstructure and Electrical Properties Evolution of Si₃N₄ Ceramics

Abstract

Si₃N₄ ceramic materials have great potential in the field of insulation in SF6 gas ultra-high-voltage transmission and transformation equipment due to their excellent insulation performance and thermal stability. In this paper, Y₂O₃-Al₂O₃ was used as a sintering aid to prepare high-density (>99%) Si₃N₄ ceramics through two-step pressureless liquid-phase sintering, and the mechanism of the influence of Y₂O₃-Al₂O₃ addition on the microstructure and electrical properties of Si₃N₄ ceramics was studied. The results showed that increasing the sintering aid content could increase the grain size of Si₃N₄ ceramics, while increasing the Y₂O₃ ratio could refine the grain size. When Y₂O₃-Al₂O₃ addition was 8% and the ratio was 5:3, the room temperature volume resistivity of Si₃N₄ ceramics was the highest, 7.33 × 10¹⁴ Ω·m, and the volume resistivity was the most stable when the sintering aid content was 12%. The internal carrier migration type of Si₃N₄ ceramics was mainly ion conduction, mainly along the grain boundaries. The temperature stability of the resistivity of Si₃N₄ ceramics could be improved by doping with Y³⁺ functional ions to reduce the potential barrier conductivity level and refine the grain size to improve the conduction path. The dielectric constant and dielectric loss of Si₃N₄ ceramics were mainly affected by interface polarization. They gradually increased with the increase in sintering aid addition. Temperature had little effect on dielectric constant and dielectric loss in the range of 20–80 °C.

1. Introduction

Ultra-high-voltage direct current transmission is an important means to achieve high-voltage, large-capacity, long-distance power transmission and grid interconnection, and it is of great significance to the construction of energy patterns [1]. Epoxy resin/Al₂O₃ composite insulators are the weak points in UHV gas insulation equipment [2,3,4]. With the increase in voltage level and transmission capacity of DC transmission systems, the electrical, thermal, and mechanical stress inside the equipment gradually increases, and the problems of thermal stability, aging performance, and surface insulation performance of epoxy resin/Al₂O₃ composite materials are prominent [5,6]. Si₃N₄ ceramics have great potential in the field of gas insulation due to their excellent insulation performance, mechanical performance, thermal stability, and chemical stability [7,8].

Currently, research on Si₃N₄ ceramics mainly focuses on mechanical properties, thermal conductivity, etc. [9,10,11,12]. Researchers promote the development of high-aspect-ratio columnar β-Si₃N₄ grains by adjusting the liquid-phase composition [13], adding β-Si₃N₄ seeds [14], increasing the sintering temperature, and prolonging the insulation time [15], in order to form prominent bimodal morphology and achieve a self-reinforcement effect to enhance mechanical properties. Kitayama et al. [16] gradually reduced lattice oxygen content and improved thermal conductivity by adjusting the Y₂O₃/SiO₂ ratio to control SiO₂ activity in the liquid phase. There is less research on the electrical properties of Si₃N₄ ceramics. Si₃N₄ ceramics are electrical insulators owing to their large band gap. However, their electrical characteristics may change depending on their microstructure [17]. Tsukaho et al. [17] studied the effect of Yb₂O₃ addition on the resistivity of Si₃N₄ ceramics; the volume resistivity was related to the internal crystalline phase of ceramics, and the resistance of Yb₂O₃ addition was decreased significantly compared with the Y₂O₃ addition of Si₃N₄ ceramics, the main reason of which is the formation of continuous Yb4Si₂O₇N₂ crystalline phase. Ceramic dielectrics materials like Si₃N₄ are reported to exhibit evidently better short- and long-term insulation performance under a DC electric field with satisfied charge suppression property [18,19]. However, a pillar insulator used in gas insulation equipment has not been reported. The SF6 gas-insulated high-voltage DC electrical equipment operating temperature is about 80 °C, and the insulation performance of the material under this condition has attracted much attention.

Si₃N₄ is a strong covalent bond compound with low self-diffusion coefficient and is difficult to be densified through solid-phase sintering. Si₃N₄ ceramics are densified through liquid-phase sintering with the addition of sintering aids [13]. These sintering aids play an important role in changing the microstructure of Si₃N₄ ceramics. Therefore, this article uses Y₂O₃-Al₂O₃ as a sintering aid to prepare high-density Si₃N₄ ceramics through two-step liquid-phase sintering without pressure. The effects of the content and proportion of sintering additives on the micromorphology and electrical properties of Si₃N₄ ceramics at different temperatures were studied and provided data support for the application of Si₃N₄ ceramic materials in SF6 gas-insulated HVDC electrical equipment.

2. Experimental Materials and Methods

2.1. Raw Materials and Experimental Procedure

Si₃N₄ ceramics were prepared using α-Si₃N₄ (SN-E10, α phase > 95%, D50 = 0.297 μm; Ube Industries, Ltd., Tokyo, Japan), Al₂O₃ (99.9%, D50 = 0.2 μm; Taimei Chemicals Co., Ltd., Tokyo, Japan), and Y₂O₃ (99.9%, D50 = 0.4 μm; Shenzhen Lanyi New Materials Co., Ltd., Shenzhen, China) powders as raw materials.

According to a certain ratio of m(Si₃N₄):m(Al₂O₃):m(Y₂O₃), the raw material powder was weighed and added to anhydrous ethanol as a medium and then placed in a Si₃N₄ ceramic jar. Small Si₃N₄ balls were added to the jar in a mass ratio of 2:1, and planetary ball milling was carried out at a speed of 300 r/min for 6 h. The milled slurry was dried on a rotary evaporator and then sieved through a 100-mesh sieve. The sieved powder was then molded under a pressure of 20 MPa to a size of Φ50 × 3 mm; then, cold isostatic pressing was performed under a pressure of 200 MPa for 5 min. The sintering process was carried out under a 0.1 MPa N₂ atmosphere, with a heating rate of 15 °C/min from room temperature to 1200 °C, then a rate of 10 °C/min to 1400 °C, a rate of 5 °C/min to 1600 °C, and a rate of 3 °C/min to 1825 °C; the samples were cooled at a rate of 10 °C/min to 1200 °C and finally cooled with the furnace. The sintering process was as follows: after holding at 1650 °C for 2 h, the temperature was raised to 1825 °C and held for 2 h. The composition and relative density of the samples after sintering are shown in

Table 1. Sample composition, sintering process, and relative density.

Sample Name	Molar Ratio			Firing Condition	Theoretical Density (g/cm3)	Relative Density
	Si3N4	Y2O3	Al2O3
1825-5Y-92SN	92	5	3	1650 °C, 2 h; 1825 °C, 2 h	3.27	99.08%
1825-5Y-90SN	90	5	5	1650 °C, 2 h; 1825 °C, 2 h	3.27	99.69%
1825-6.25Y-90SN	90	6.25	3.75	1650 °C, 2 h; 1825 °C, 2 h	3.28	99.70%
1825-6.8Y-90SN	90	6.8	3.2	1650 °C, 2 h; 1825 °C, 2 h	3.29	99.40%
1825-7.5Y-88SN	88	7.5	4.5	1650 °C, 2 h; 1825 °C, 2 h	3.30	99.39%

.

2.2. Performance Testing and Characterization

The sintered Si₃N₄ ceramic was ground and polished; the bulk density was measured using the Archimedes drainage method; the theoretical densities of the Si₃N₄ ceramics were calculated depending on the composition and density of the raw materials; and the relative density was calculated based on the theoretical density and bulk density. The relative density of all samples was greater than 99%. The phases present in the samples were identified by employing X-ray diffraction (XRD; D8 Advance; Bruker, Ettlingen, Germany) using Cu Kα radiation at an acceleration voltage of 40 kV, tube current of 20 mA, and scanning range of 10−60°. After grinding and plasma-etching the ceramic, the microstructure was observed using a high-resolution field-emission scanning electron microscope (SEM; Nova, NanoSEM430; FEI, Eindhoven, The Netherlands). The volume resistivity of the composite material of Φ30 mm × 1 mm was measured using the three-electrode method with a Keithley-6517B high-resistance meter; the voltage was 1 kV, and the temperature range, 20−80 °C in air. The dielectric constant and dielectric loss of the sample were tested using the German Novocontrol Concept80 wide-frequency dielectric impedance spectroscope at room temperature and a testing frequency of 10⁰~10⁵ Hz.

3. Results and Discussion

Figure 1 shows the XRD spectrum of the sintered Si₃N₄ ceramics with different Y₂O₃ and Al₂O₃ contents. It can be seen that main peaks are characteristic β-Si₃N₄ peaks for all samples, which indicates that the α phase completely transformed into β phase during sintering. The width of the β-Si₃N₄ peaks is narrow; the diffraction peak is sharp; and there are no other mixed peaks, which indicates high crystallinity of β-Si₃N₄. However, when the sintering aid content increased to 12%, in addition to the β-Si₃N₄ peaks, low-intensity peaks characteristic of Y₂SiAlO₅N were detected. The Y₂SiAlO₅N crystalline phase formed through the reaction of the added sintering agent with SiO₂ on the surface of Si₃N₄ powder.

Figure 2 shows the etched surface and sectional microstructure of sintered Si₃N₄ ceramics and some EDS spectra. It can be seen from the surface-etching diagram that all samples have dense surfaces without obvious pores. The main phase is black, while the grain boundary phase is white. The grain boundary phase is the nitrogen oxide compound generated by the reaction of sintering additives and silicon dioxide on the surface of Si₃N₄ powder at high temperature, which is distributed in amorphous or partially crystalline form among grains or polycrystalline junctions after cooling. The microstructure of the samples is uniform and contains fine equiaxed Si₃N₄ grains and large rod-like Si₃N₄ grains. The two types of grains form an interwoven network structure, which is called self-reinforcing bimodal structure. From Figure 2a,c,k, it can be observed that with the increase in sintering aid content, the grain size of Si₃N₄ ceramics increased. This is mainly due to the higher liquid-phase content during the sintering process of Si₃N₄ ceramics with higher aid content, resulting in more uniform liquid-phase spreading and easier precipitation and growth of β-Si₃N₄ in the liquid-phase-enriched region [20].

From Figure 2c,e,g, it can be seen that when the sintering aid content was the same at 10%, the grain size of Si₃N₄ ceramics gradually decreased with the increase in Y₂O₃ proportion. This is mainly because an increase in Y₂O₃ proportion leads to higher liquid-phase formation temperature and increased viscosity of the liquid phase, resulting in an increase in nucleation sites and finer β-Si₃N₄ grains due to competition for growth materials and spatial hindrance effects. Figure 2i,j shows the distribution of Y and Al elements in sample 1825-6.25Y-90SN. The elements Y and Al are mainly distributed along the grain boundaries. The sectional morphology is similar to the surface and consists of a large number of interlocking β-Si₃N₄ columnar crystals. With the increase in sintering aid content, the grain size of Si₃N₄ ceramic increased, and the distribution of columnar crystals became more uniform. However, when the ratio of Y₂O₃ to Al₂O₃ was 6.8:3.2, the liquid-phase concentration increased, and local defects formed due to spatial hindrance. Therefore, when preparing high-density Si₃N₄ ceramics, not only the total amount of sintering aids should be considered, but also the composition of sintering aids should be focused on. The use of sintering aids with different rare earth/alkaline earth ratios can achieve phase transformation and densification regulation, thereby controlling the microstructure of Si₃N₄ ceramics, which is particularly important for improving the mechanical and electrical properties of Si₃N₄ ceramics [21].

The distribution characteristics of the direct current electric field are closely related to the conductivity characteristics of insulating materials, and the conductivity and temperature characteristics of insulating materials dominate the distribution of the electric field in insulating structures. The volume resistivity change characteristics of Si₃N₄ ceramic materials under a field strength of 1 kV/mm in the temperature range of 298–353 K were tested using the three-electrode method, as shown in Figure 3. The volume resistivity of Si₃N₄ ceramic composite materials at room temperature (298 K) and under an electric field of 1 kV/mm was greater than 10¹⁴ Ω·m. The room temperature resistivity of sample 1825-5Y-92SN was the largest, 7.33 × 10¹⁴ Ω·m, significantly better than the value of 10¹¹ Ω·m obtained by Lukianova, O.A. et al. [18]. With the increase in sintering aid content, the resistivity of Si₃N₄ ceramic composite materials gradually decreased, while the proportion of sintering aid had little effect on resistivity. With the increase in temperature, the decreasing trend of the volume resistivity of Si₃N₄ ceramics gradually slowed down. Compared with the values at room temperature, the volume resistivity of the five samples was between 35% and 50% at 80 °C. Sintering aids were beneficial to improving the temperature stability of Si₃N₄ ceramics. The volume resistivity was the most stable when the aid content was 12%, with a room temperature volume resistivity of 4.40 × 10¹⁴ Ω·m and an 80 °C volume resistivity of 2.13 × 10¹⁴ Ω·m, with a remaining rate of 48.4%. The main reasons are that the doping of functional ions, such as Y³⁺, can introduce weakly bound ions and lower the potential barrier conductivity levels in the material and the formation of Y₂SiAlO₅N crystalline phase reduces the carrier concentration and reduces the influence of temperature changes on the volume resistivity of the material. When the sintering aid content was 10% and the ratio of Y₂O₃ to Al₂O₃ was 5:2.35, the volume resistivity had the best temperature stability at 47.4%, decreasing from 5.59 × 10¹⁴ Ω·m at room temperature to 2.65 × 10¹⁴ Ω·m. In addition to the aforementioned reasons, this method also reduces the grain size of silicon nitride ceramics and prolongs the carrier migration path. In contrast, the volume resistivity of epoxy resin/Al₂O₃ composite materials used in current ultra-high-voltage transmission and transformation equipment decreases from 10¹⁴ Ω·m to 10¹² Ω·m when the temperature rises from 298 K to 353 K, with a decrease of more than 99%. It is obvious that Si₃N₄ ceramics have better temperature stability and significant advantages in improving the distribution characteristics of the electric field under temperature gradients in practical applications.

Si₃N₄ ceramic materials mainly exhibit ionic conductivity in terms of carrier migration. The Si₃N₄ ceramic materials prepared in this study have a density close to the theoretical density, and the impurity ions mainly consist of a small amount of glass-phase residue of sintering aids in the grains. The general expression for conductivity is as follows:

$$\sigma =N\times \frac{q^2{\delta }^2}{6kT}\times {\nu }_0\exp (-W/kT)$$

(1)

where N represents the logarithm of ion pairs per unit volume; q represents the charge of the ion; δ represents the distance between adjacent semi-stable positions; k represents the Boltzmann constant; T represents the thermodynamic temperature; ν₀ represents the vibration frequency of impurity ions at semi-stable positions; W represents the activation energy of conductivity.

The conductivity depends on the carrier concentration and mobility. The carriers in Si₃N₄ ceramics mainly come from impurities among grains, which are residual glass-phase materials remaining at the grain boundaries after sintering. The migration of carriers is also closely related to the process of ion diffusion, according to Fick’s first law.

$$J=-D\frac{\partial c}{\partial x}$$

(2)

where J represents the diffusion flux; c represents the solute concentration; x represents the coordinate in the diffusion direction; the diffusion coefficient D is a fundamental parameter that describes the diffusion process, with unit m²/s. The relationship between D and ionic conductivity is

$$\frac{\sigma }{D}=\frac{n{q}^2}{fkT}$$

(3)

where n represents the number of ions per unit volume; q represents the charge of the ions; f represents the correlation coefficient; T represents the thermodynamic temperature.

Therefore, the migration behavior of carriers in Si₃N₄ ceramic materials can also be explained by ion diffusion behavior. In these materials, the diffusion path of ions has an important influence on diffusion behavior. Diffusion inside a normal crystal is collectively referred to as lattice diffusion, volume diffusion, or bulk diffusion. In contrast, diffusion along dislocations, grain boundaries, or surfaces is called dislocation core diffusion, grain boundary diffusion, or surface diffusion, respectively. Diffusion along line defects and surface defects is generally faster than bulk diffusion, so this phenomenon is referred to as “short-range diffusion” in the literature, referring to diffusion that occurs along paths that are much faster than bulk diffusion, including dislocations, grain boundaries, and solid-phase boundaries [22]. In summary, on the one hand, under the action of an electric field, the diffusion rate of carriers in Si₃N₄ ceramics is slow, causing carriers to tend to migrate along the grain boundaries. On the other hand, the “short-range diffusion” path is determined by the direction of the grain boundary, which does not completely coincide with the direction of the electric potential gradient, resulting in a decrease in the electric field force and an increase in the migration path during the actual migration process of carriers, thereby resulting in higher resistivity of the ceramic body. At room temperature, impurity ionization inside the ceramic provides carriers for the conduction process, and the carriers mainly migrate along the grain boundaries. With the increase in temperature, more impurity ions are ionized from the glass-phase impurities. At the same time, the lattice vibration of the ceramic intensifies, but its basic framework structure remains unchanged; so, the migration path of carriers is less affected, and most carriers still migrate along the grain boundaries. The increase in temperature increases the carrier concentration inside the material and enhances the migration speed of carriers along grain boundaries, with little influence on the migration path of carriers. This is manifested as higher temperature stability of the resistivity of Si₃N₄ ceramics at the macro level. The migration process of carriers inside ceramics is shown in Figure 4.

The relationship between the dielectric constant and frequency of Si₃N₄ ceramic materials is shown in Figure 5. When the ratio of Y₂O₃ to Al₂O₃ in the sintering aid was 5:3, the dielectric constant of Si₃N₄ ceramic materials gradually increased with the increase in sintering aid addition. At a frequency of 50 Hz, the dielectric constants of the 1825-5Y-92SN, 1825-6.25Y-90SN, and 1825-7.5Y-88SN samples were 7.89, 8.35, and 8.50, respectively. The ratio of sintering aid had little effect on the dielectric constant of Si₃N₄ ceramic materials. With the increase in frequency, the dielectric constant of the composite material slowly decreases, which is mainly related to interface polarization. The polarization of Si₃N₄ ceramic composite materials under the action of an electric field is not instantaneous but has a certain polarization time. Therefore, interface polarization plays a dominant role at low frequencies. With the increase in frequency, interface polarization cannot follow the changes of high-frequency electric fields, and interface polarization gradually disappears, resulting in a gradual decrease in the dielectric constant of the composite material. The relationship between dielectric constant and temperature of Si₃N₄ ceramics at 50 Hz is shown in Figure 6. It can be seen that the dielectric constant of Si₃N₄ ceramics gradually increased with the addition of additives at 50 Hz, but the proportion of Y₂O₃ to Al₂O₃ had little effect on the dielectric constant at 50 Hz. In addition, the temperature range of 20–80 °C had little influence on the dielectric constant of Si₃N₄ ceramics; for example, the dielectric constants of sample 1825-7.5-Y-88Sn at 20–80 °C were 8.50, 8.53, 8.55, and 8.56, respectively. The dielectric constant increases slightly with the increase in temperature, which may be caused by ion migration at high temperature. Liu et al. found that the dielectric constant of Si₃N₄ ceramics was mainly affected by the content of additives and density rather than the crystallinity of the second phase. In the experiments, the density of all samples was higher than 99%, so the dielectric constant of the samples hardly changed [23].

The relationship between the dielectric loss and frequency of Si₃N₄ ceramics is shown in Figure 7. It can be seen that when the frequency was greater than 100 Hz, the dielectric loss increased with the increase in sintering aid content. This is mainly due to the increase in sintering aid content, which increases the carrier concentration inside the ceramic material. At low frequencies (less than 50 Hz), the dielectric loss of the composite material is mainly affected by interface polarization and the distribution state of sintering aids. Similar to the dielectric constant, interface polarization plays a dominant role at low frequencies. From the previous microscopic morphology of Si₃N₄ ceramic material surfaces, it can be seen that when the ratio of Y₂O₃ to Al₂O₃ was 5:5, due to the higher content of Al₂O₃, abnormally large β-Si₃N₄ grains were formed. When the ratio of Y₂O₃ to Al₂O₃ was 5:2.35, the content of Y₂O₃ was higher; the grains were refined; and more interfaces were formed between Si₃N₄ and sintering aids, resulting in an abnormal increase in dielectric loss. The relationship between the dielectric constant and temperature of Si₃N₄ ceramics at 50 Hz is shown in Figure 8. Similar to the dielectric constant, it can be seen that the temperature range of 20–80 °C had little influence on the dielectric constant of Si₃N₄ ceramics. Through the analysis of the dielectric properties of Si₃N₄ ceramic materials, it can be seen that Si₃N₄ ceramic materials have a large dielectric constant; low dielectric loss; easy-to-control dielectric properties; and strong temperature stability, which is especially suitable for the use of ultra-high-pressure gas insulation environments.

4. Conclusions

This article investigates the correlation between the microstructure and electrical properties of Si₃N₄ ceramics, exploring the potential of using Si₃N₄ ceramics to enhance the surface insulation performance of SF6 high-voltage direct current electrical equipment. The following conclusions are mainly drawn: (1) Y₂O₃-Al₂O₃ was used as a sintering aid to prepare high-density (>99%) Si₃N₄ ceramics through two-step pressureless liquid-phase sintering. With the increase in sintering aid content, the grain size of Si₃N₄ ceramics increased; when the aid addition amount was 10%, the grain size of Si₃N₄ ceramics gradually decreased with the increase in Y₂O₃ proportion. (2) The room temperature resistivity of sample 1825-5Y-92SN reached a maximum of 7.33 × 10¹⁴ Ω·m; the increase in sintering aid content decreased the volume resistivity of Si₃N₄ ceramics but improved the temperature stability of volume resistivity; when the aid content was 12%, the volume resistivity was the most stable, with a remaining rate of 48.4% at 80 °C. (3) Under a DC electric field, the carrier migration type of Si₃N₄ ceramics was mainly dominated by ionic conductivity, mainly along the grain boundaries; by doping with Y³⁺ functional ions to reduce the potential barrier conductivity level and refine the grain size, the temperature stability of the resistivity of Si₃N₄ ceramics could be improved. (4) With the increase in sintering aid content, the dielectric constant and dielectric loss of Si₃N₄ ceramics gradually increased, mainly influenced by interface polarization, at a frequency of 50 Hz; when the sintering aid content was 12%, the maximum dielectric constant was 8.50.