Crystallization, Dielectric, and Energy Storage Properties of Phosphate Glass–Ceramics

Introduction

In recent years, high power density dielectric capacitors with ultrafast charge-discharge speed have seen extensive applications in pulse power systems such as those used in the military sector, hybrid automobiles, and for the storage of renewable energy [1– 3]. The low energy storage density of dielectric capacitors limits their further development with respect to fuel cells and batteries. [4,5]. Hence, increasing the dielectric energy storage density is now a serious obstacle for scientists to surmount. Dielectric ceramics and polymer-based dielectrics are the two primary types of energy storage materials [6,7]. Although dielectric ceramics have a high permittivity, they generally display a relatively low breakdown strength (BDS) [8,9]. Conversely, polymer-based dielectrics generally provide high BDS, but they maintain low dielectric constant and poor thermal stability [10,11].

In this regard, the major emphasis of research has been on dielectric glass-ceramics, which are characterized as composite materials that combine the benefits of ceramic phase and glass matrix [7]. Crystal phases could substantially improve the permittivity, and glass networks can enhance BDS. [12,13]. Despite being classified as linear dielectric materials, glass- ceramics have an energy storage density related to the square of the BDS and the dielectric constant [1,7]. In the present glass-ceramics system, the formation of the ceramic phase is constrained by the dense glass network structure, which results in a high BDS and low permittivity. As a result, the achievement of an optimum energy storage density is limited. Hence, it is more likely to enhance both the dielectric constant and BDS. The emphasis of glass-ceramics research focuses primarily on niobate-based glass systems [7,14]. By controlling the content of the glass forming and the precipitation phase during the crystallization process, the grain size and content of the ceramic phase with high dielectric constant can be efficiently controlled, so the energy storage may be improved [15,16]. Wang et al. investigated the influence of nucleating agents (CeO2, ZrO2, and CaF2) on the energy storage density of KSN-based glass- ceramics. They revealed that the addition of 3 mol% CaF2 promoted the precipitation of the crystalline phase with high permittivity, and they optimized the dielectric constant and breakdown strength [17].

Recently, (BaO,Na2O)-Nb2O5 with different glass phase formation has been studied from the phase ratio, crystallization time, interface polarization, and discharged property [15,18]. Likewise, the permittivity of glass-ceramics can be increased by adopting a higher crystallization temperature. Tao Jiang et al. reported the effect of crystallization temperature on the phase development of 21.25BaO–1PbO-12.75Na2O–34Nb2O5– 32SiO2 and reported that the relative permittivity progressively rose due to the formation of phases with high dielectric constant [19]. Kaikai Chen reported that when BaO-Na2O-Bi2O3- Nb2O5-Al2O3-SiO2 glass- ceramics were crystallized at higher temperatures, the discharged energy density increased, reaching an optimal discharge energy density of 0.48 J/cm3 for the sample crystallized at 950°C [20]. In our previous findings, we showed. that increasing the crystallization temperature improved the dielectric permittivity and the energy storage density of BaO- NaO-Nb2O5-P2O5 glass-ceramics [21].

Moreover, Ihyadn et al investigated the effect of WO3/Nb2O5 on the performance of a BaO- Na2O-Nb2O5-P2O5 glass-ceramic system and found that raising the WO3 content resulted in high energy efficiency [22]. In present work, BaO-Na2O-Nb2O5- WO3-P2O5 (BNNWP) glass- ceramics were crystallized at different temperatures. The phase evolution, microstructure, dielectric performances, impedance spectroscopy, and energy storage properties were investigated in various heating temperatures in order to find the optimum crystallization temperature of BNNWP glass-ceramics.

Experimental Procedure

The33.3BaO-8.3Na2O-33.36Nb2O5-8.34WO3-16.7P2O5 (BNNWP) glass system was synthesized via the quenching method as reported in our previous work [22]. In addition, the BNNWP glasses were crystallized at different temperatures 760°C, 800°C, 900°C and 1000°C, and designated as B760, B800, B900, and B1000 respectively. The samples were crystallized in the air for 10 h with a heating rate of 10 â?¦C/min based on DSC results.

The characterization techniques used for investigating the structural, microstructural, and dielectric properties were cited in our previous work [21]. The energy storage properties were studied using the CPE1701, PolyK, USA, with a high voltage power supply (Trek 609-6, USA). For the P–E measurement, silver electrodes with a 13 mm diameter and 0.25 mm thickness were coated on the two sides of the samples.

Result and Discussion

Phase Evolution and Microstructure

The DSC result of the BNNWP glass sample acquired at a heating rate of 10 °C min is shown in Figure 1. Additionally, two exothermic peaks, Tp1 and Tp2, are seen on DSC plots. Tp1 is located at 735°C, while Tp2 is around 827°C. Such two anomalies are evidence that two ceramic phases within the glass matrix were formed.

Figure 2 depicts the phase evolution of BNNWP glass-ceramics at various crystallization temperatures. According to the XRD patterns, two main phases were formed: Ba2NaWNb4O15, which is a tetragonal tungsten bronze structure, and NaNbO3 in which the perovskite structure is cubic. Additionally, it is noticed that the peaks intensity of XRD analysis increases as the crystallization temperatures rise. These findings may suggest an increase in the dielectric constant as the heat treatment temperature rises. Similar results have been observed elsewhere [14,19].

Figure 3 displays SEM images of BNNWP glass-ceramics. The microstructure of the glass- ceramics is homogenous and dense, with regular grain shapes and uniform grain size distributions. In addition, the white particles observed for B1000 could be attributed to impurity phase emerging at a high crystallization temperature. It is observed that the average grain increases as the crystallization temperature increases. Glass-ceramics crystallized at 760°C, 800°C, 900°C and 1000°C present an average grain size of 1.3µm, 1.5µm, 1.7µm, and 2µm, respectively. The average grain size was calculated by Image J. Large grain sizes can promote polarization, which can improve the dielectric constant. However, it may result in a loss in BDS because of charges accumulating at grain boundaries as a result of the huge disparity between large grain size and glass matrix [23].

Dielectric Investigations

The thermal variation of the dielectric constant and dielectric losses of BNNWP glass-ceramics is illustrated in Fig. 4. It is evident that an increase in crystallization temperature increases the samples' dielectric constant. For instance, it rises from 65 to 100, and 218 and 315 at 760°C, 800°C, 900, and 1000°C, 

respectively. This result could be explained by an increase of ferroelectrics phases Ba2NaWNb4O15 (εr=100-300) and NaNbO3 phases (εr= 500-600) with highεr, as the crystallization temperature increases according to XRD results [19,20,24- 26]. Furthermore, it should be noticed that this rise correlated with increased of the grain size detected in the SEM results. In addition, the significant thermal stability of the dielectric constant with a rate of variation less than 3% was noticed in the temperature range of 25°C to 250°C for all samples except for B1000. Dielectric capacitors are typically employed at various ambient temperatures. The outstanding thermal stability of this material is therefore of great practical importance for technical applications. Furthermore, the dielectric loss reduces with increasing crystallization temperature and eventually increases for B1000. It passes from 0.017 to 0.008, and 0.011, and 0.038 for B760, B800, B900, and B1000, respectively at Room temperature. Despite this, the dielectric loss remains less than 0.04 for all the samples due to the defect-free quality of the glass phase.

Impedance Spectroscopy

Figure 6 displays the complex impedance spectrum of B760 glass-ceramic from 400°C to 480°C, with a range of 20°C. A simple parallel resistance-capacitance R-C circuit was used to simulate the contributions of the grain and grain boundary of the samples. The area of the curve declines as the test temperature increases, revealing that the impedance of the glass-ceramic decreases. This could be attributed to the thermal activation of defects generated by Maxwell– Wagner interface relaxation originating from the interaction of grains and glass matrix [28]. As illustrated in the inset of Fig. 6, the equivalent circuit is represented utilizing two R-C elements connected in series. ZVIEW® was deployed to fit our experimental results.

Conclusion

In conclusion, the temperature of crystallization has a considerable effect on dielectric characteristics, energy storage properties, phase formation, and microstructure evolution. The precipitation of Ba2NaWNb4O15 and of NaNbO3 phases from the glass matrix was revealed by XRD analysis. In addition, the content of the forming phases increased as the crystallization temperature increased. The relative permittivity increases gradually with the crystallization temperature. The dielectric constant at room temperature increased from 65 to 315 as the heat treatment temperature increased from 760 to 1000 °C. This behavior was connected to the proportions of the phases and the incase in average grain size of BNNWP glass-ceramics. The dielectric study revealed that the samples showed good temperature stability. For the crystallization temperature range of 760 °C to 900 °C the dielectric constant varies by less than 3% range of 25° to 250°C. The sample heated at 1000 °C has a high Wrec of 177mJ/ cm3, combined with a dielectric constant of 315.

Acknowledgments
The authors gratefully acknowledge the financial support of CNRST, OCP foundation, and the European Union’s Horizon H2020-MSCA-RISE research and innovation actions,-ENGIMA and MELON.

References